I guess I'm qualified to answer this. Unlike some of the other answerers here, I was not born with any particular physical defect (barring a slight lisp which I've managed to iron out). I was simply born - quite simply - ugly. I'm 23, and have never got anywhere with a girl. Never been kissed, let alone any further.

Especially as a man, you feel there's something wrong about you. At my secondary school, I was one of the more popular kids. I was an extremely talented footballer (I am justified to say that, I once had trials with Rangers FC). There was a serious locker room culture in the team. Everyone acted like a stereotypical man, talking constantly about sex, who the hottest girl was that they'd ever got off with etc was. And I was just sitting there, not knowing what to say because I'd never experienced that. In my group of friends too. They were all nice, kind people. Not nasty at all. But when it came to parties, they would all be getting off with people...except me, I was all alone. I began to feel like there was something wrong with me, that I wasn't good enough. Because it's easier to be a girl (not easy though) and still be a virgin, not get off with anyone either, as people just say you're saving yourself up for 'the one'. As a guy, you're expected to get with girls as much as possible. I know that was a flawed view. But this was secondary school when peer pressure etc was immense.

You get used to disappointments. So when we had a prom at my secondary school, I was one of maybe 3 kids out of a year of 120 kids, who didn't have a date to prom. But I wasn't surprised by that. The fact that all the girls I asked to prom said 'no' didn't surprise me at all. I'd already been expecting that a long time previously. Same sort of thing when I was a student, queuing up for nightclubs. The bouncers would split up the groups...and guess what? I would be the one told the club was already full, and would have to go home on my own

You feel one of two ways about yourself - either you constantly fret and get depressed about your lack of looks. Or you don't give a flying fuck about what people think you look like. I felt different ways at different times. When I was younger, a tweenager, even up to 20ish, I constantly fretted and was depressed about it. I would look in the mirror and call myself ugly. I wore my hood up in public because I felt people would laugh at me if they saw my face. I knew I was overreacting, but what could I do? People trotted out the corny line "be patient, there's someone for everyone" but I wasn't convinced even then - and I'm definitely not now. I took to drinking I became so depressed. At the lowest point in my life, I could easily get through a litre bottle of vodka in maybe 2-3 hours and show no obvious side effects. But after that low point, I changed the other way, to not giving a damn. People will do this in two ways. Either they will not take any care in how they dress, won't wash regularly, won't bother with anything in their personal appearance. Or, as other people such as I do, we make the best of a bad job: dress well, keep clean and tidy, and ignore other peoples' views and stop worrying how others think of you. but we have a rock hard shell. We grow immune to other peoples' comments. You could pretty much give me any insult nowadays to do with my looks and it would bounce off me - I've heard them all, some several times over

You feel terrified around anyone goodlooking. Less various of my friends - but especially with anyone I know less well, be it a friendly acquaintance, business acquaintance or simply a pure stranger - if they speak to you, you undergo lots of different feelings. Embarassed and extreme humility that this goodlooking guy/girl is talking to you, defensiveness in case they follow their words up with an insult - and simply personal shame. That's what really gets me, how I feel. I have seen from answers on the corresponding question to this that other people act the same - this is not just me who feels so ashamed and embarassed at a good looking person talking to me.

You become instantly suspicious of any pretty girl who seems in the slightest bit friendly to you. The fact she might simply be trying to be friendly is repressed in your mind. As soon as you get into an idle conversation with the pretty girl next to you at the bar - about how long the queue is, or whatever - you are instantly on your guard, expecting her to try to use you for a free drink, or whatever. The being used for drinks has never happened to me - but maybe that''s because I'm automatically cautious as soon as anyone remotely pretty talks to me.

You feel helpless sometimes, angry at why you were born that way . Because ugliness can't be changed. You can't get an erection? Cool, viagra. Your boobs too flat? Cool, boob job. Overweight? Cool, go to the gym.  Wrinkles? Cool, facelift and botox. Simply ugly? Tough shit, nothing you can do to change it - especially as a man. Women can change their appearances suprisingly well with makeup: men don't have that luxury.

You really hate the words "you're imagining it!", "it's all in your head", or "I'm sure someone finds you attractive. Only strangely, whenever you quiz that person further, they don't know anyone who thinks that way. They don't. Their friends don't. But they're sure someone must. And when people say it's in your head. Really? Because for 10 years now I've been this way. Not once has any woman shown any interest in me. I'm not bitter at them for not doing so - it's just clear proof that it's not that it's "all in my head". It's not I expect a girl to have the face of a supermodel and killer curves. That's pretty low down on my list. I'm much less superficial than you might think. But I know and have proof I'm ugly - why else would absolutely noone show any interest in me? I understand why people say it. hey feel awkward, realise they must say something - but don't really know what to say. Please don't get the idea I always moan on about this to other people, it's the very few people I've confided this in who've trotted out this line.

Humour will only get you so far. As Marilyn Monroe said "if you can make a girl laugh you can make her do anything". Not true. You might be able to make the girl laugh - but the good looking guy will be able to do much more easily, and make her smile too. Because she won't just go for your humour. In this society, like it or not, superficial looks are what first count. If you get past the first hurdle as you are sufficiently good looking, your partner might also like your personality. But what first counts is your looks.

Double standards. I was raised as a child to always be polite to a woman. That means opening the door for a woman, giving up your seat for her on a train. I do those things as I believe they are right. I don't use them to try to hit on girls. But when I, as an ugly guy do it, the girls either look at me as a weirdo and decline - or inwardly laugh and think "loser, trying to play at being a gentleman to get with me. Pathetic, haha". But if your good looking guy does the same, the girl is instantly attracted to him, even though he's given up his seat for the same reason as I have, out of basic politeness.

Frustration. Because you think you can do so much else, just not that. I am seriously good at football. I'm ripped with a six pack. My score last week was 16.8 on the bleep test. I have an IQ of 120 and ran the London Marathon in 3 hours. I read a lot too - not James patterson, but Tenessee Williams and Shakespeare for plays, William Golding and JG Ballard, all the english classics. I'm well off with two flats and a car. I have plenty of friends and a social life. I know, why should I therefore be so upset over my ugliness? And I get angry at myself, halfway between hating myself for being so ungreatful to life, and half the time frustrated that if I could achieve all these other things, why the hell can't I achieve a much more common one that almost everyone has achieved at some point in their life?

You know what? At first I was going to put this answer on anonymous because I was ashamed how it would sound. But then I realized that I really shouldn't care two hoots about it anyway.

What are the advantages of my situation?

Advantages
It's much easier to make both male and female  friends. The women , although they won't find you attractive, will see  you as a good friendGuys won't view you as a 'threat' when they are trying to chat up girls,  so again, it's easier to make friends with them.I'm actually quite a decent wingman
People in  terms of work will take me seriously for my capabilities, and not for my  looks.due to my relative lack of attractiveness, people will never  assume that I got the job by sleeping my way to the top. Especially in  my last two jobs, as a soldier and then as a private security  contractor, my ugliness fitted the job. People are more suspicious  almost of good looking soldiers and contractors - they expect s to be,  although muscley and high endurance, not at all good looking. I fitted  that stereotype perfectly. That has worked in my favour, as people are  more likely to give contracts to my firm in terms of contracting, as, I  have been told by contractors and civillians alike, my face fits the  idea many have of contractors.
I don't get hit on constantly.  In fact, I never do. That's good though - it means if I am going on a  night out to a club, or a bar, I know I won't have to worry about  brushing off advances the way that many attractive women do, and some  attractive men do as well.
I am appreciated for my personality.  People listen to what I have to say, actually take it in, enjoy my  personality. If I was attractive, I might have the problem people  concentrated mainly on my looks, without bothering to look further at my  personality
Nothing much is expected of you. I look the same pretty much every day. I dress well, designer clothes et al, but no one  expects me to come in looking like a supermodel. So I don't need to  freak out do badly if I cut myself slightly while shaving, or get a spot  on my face - it won't make a difference. Some of my better looking guy  friends have these worries, I don't.
Being ugly is inevitable.  As I am ugly, I don't need to worry about getitng ugly. I already am.  But those who are good looking definitely do worry more, constantly,  about losing their looks. As Lord Henry says in A Portrait of Dorian  Gray, "what the gods give [Dorian's good looks], they quickly take  away". Unlike attractive people, I never need to freak out about  becoming ugly the older I get!
Although I'm well off, I'm not  stinking rich either. So when I get a new friend, I know they really are  my friend because of me - they are not in it for my money or my looks. I  never need to worry about that
I've grown a thick skin over time, from everything that has (not) happened. As mentioned earlier, jibes about my looks won't really affect me anymore. I no longer fret about my looks at all. I keep clean and tidy and dress well, but that's it
I don't get judged so much  .Eye-pleasing  people tend to get judged by those less attractive in the  world. Men  automatically hate a handsome man, because we’re a  competitive gender,  and the good-looking guy has an advantage in the  situation. Women  automatically make assumptions about an attractive guy  - he’s probably  conceited/a womanizer/gay. Men may automatically  assume an attractive woman is bitchy/stuck up. Dunno about women's  instant view of attractive women .But you see someone ugly, you simply  think - if anything at all - "He's ugly" and that's that. You don't  promptly assume the guy is nasty or mean or anything. So I'm not judged  before I've even had a chance to talk to someone - that's a pretty  decent advantage to me
self acceptance is something you  learn to have. Because it's the only thing that really made me OK about  my looks. because the whole point is that you shouldn't get worried, as  there's nothing you can do to change it. A lot of the commenters have suggested good things I should try. I have tried almost all of them. One I tried and didn't like, however, was the thing that every morning you should compliment  yourself, say aloud to yourself that "I am a beautiful person". But this  is self-deception. And you know it. When  you say, "I am beautiful on the inside", deep down  inside you really know  what that means, "I am a disgusting piece of  repulsive meat that no one  wants to sleep with". You cannot be happy  with self-deception because  you will truly know what you are. So the  first step of overcoming your  depression over your appearance is to tell  yourself, "I am a disgusting  piece of meat" and accept yourself for  what you are.When you have this  level of self acceptance, as I have learned over years to get, your  worries about looks fade away. Beautiful people don't have that  advantage though. They always have to worry, and are never happy with  their fading looks. Hell, I've got enough going for me in my life that  now, looks don't bother me really.
I can laugh when I hear about people having huge rows with partners, messy divorces, hating life now they have children. I have none of those worries

Disadvantages?

Everything mentioned above really - especially when I was younger
You feel a misfit that you have never known love of any sort
You hate the cinema/coffee shops/romcoms because you always see couples there, which kinda depresses you
Especially prom-wise, it really is dreadful being one of three who doesn't have a date to it. Those were probably the worst two hours of my life
It can affect you really badly, to the affect of extremely heavy drinking
You become convinced if anyone good looking talks to you, they are trying to manipulate you for something
If you like going out (as I do), it begins to suck at nightclubs if all your friends have found girls, and yet you are alone
Extreme cynicism
Loneliness. I have lots of friends, but even they have romantic lives. You can't go out with friends every night. Sometimes they will be on dates/nights in. And that's just the wekeend. Every working day, I get back to my flat. And it's empty. There's no one there to welcome me in, nothing like that. I'll eat dinner on my balcony looking out onto Hong Kong harbour. It'll be a lovely meal and a lovely view, but I have no one to share it with. That bites. When I go away (be it I'm going to be a contractor in another country, as happens most weeks, my packing takes all of 5 minutes. And I need to leave one note, to the porter about keeping post for me. My friends will know from facebook, but there's no one at home I say good bye to. No wife or kids to hug goodbye to, or anything like that. When I return to my flat, it's as empty as I left it, no welcoming voice for me. If I eat out, and it's not with mates, then I am eating alone in a restaurant. I get a lot of looks as a result. Some of pity, some embarassing times when the waiter has been astonished I only want a table for one, once I have even had a lady come up to my table and say she's so sorry I was stood up by my girl and I deserve and will get much better! So the loneliness hurts. At the end of the day, lying in bed, I'm on my own. No one to chat with or anything like that.