There is a LOT to tell, but at the same time I don't want to put down a wall of text, I want to keep it brief, simple and easy to understand. I'll try my best and cut it up into brief bits... I know this is long but LSD is so incredibly complex.

It all starts with serotonin. Serotonin is a hormone, a neurotransmitter, that literally regulates everything that is central in human existence. It is so common it is even found in single-celled organisms, and natural psychedelics like DMT and bufotenin are made from the same building blocks as serotonin.

Serotonin regulates what information passes from deeper parts of our brain to our conscious minds, it regulates our hunger, it is released in large amounts when there is a lot of positive social interaction (a sort of affirmation of your social existence), it gives us empathic feelings, it can make us incredibly horny, and it just makes us, feel alive. It's called the happy hormone, but it doesn't make you directly happy, it just makes you appreciate things simply because of your ability to experience them, because you're alive, and that makes you naturally happy.

The reason LSD has such an impact on us, is because it interacts with the receptors for serotonin. The receptors are the parts in the cell which serotonin fits into like a key, through which it can change the interior composition of the cell, sending it to different parts of the brain and body with different functions and effects. LSD locks into the receptor for serotonin but does very different and very strong things.

LSD sends a signal to your cells to "open the floodgates". Instead of regulating information in the brain, LSD just makes everything connect, all the parts of the brain start interacting and neurotransmitters involved in decision-making and focus (dopamine and norepinephrine) are released as well. This is what causes the connectivity between things that is observed and the "obvious solutions" you come to. Something that causes the hallucinogenic effects and causes things to manifest is the release of glutamate as a result of LSD interacting with the receptor, glutamate is a compound that mediates signals in the brain, it's one of the elements that drives our cognition and the actual exchange of information in the brain.

The reason your vision is distorted, colours are more intense, music is amazing, taste is enhanced, etc. is not because your senses themselves change, it's because your sensory input isn't routed through just your sensory areas, it's routed through your creative centre, your memory centre, etc. So you see more depth and dimension and richness to everything in your vision because more parts of your brain are involved in analysing the input. Additionally, many of the things you experience are a result of your visual areas themselves. You don't just see input, you see how that input is transformed into an actual visual experience. Geometric hallucinations are a direct product of the way your cells are arranged in certain patterns in the visual areas of your brain.

Likewise, you can relive memories with incredible vividity because they become directly accessible to your conscious mind. At the same time you can use other parts of your brain to observe them and that is why you get amazing insights about yourself, especially if those memories are recalled because you are looking at photos/videos of your past self. You can also think something and while you are thinking it, observe your thoughts and where they come from.

You feel one with the universe because you don't observe it from multiple "angles" (through vision, smell, thought, etc.) but all these senses, emotions and thoughts blend into one single sensation because everything just goes criss-cross in your mind, instead of using different specific areas in your brain to observe and process information, you use your brain as a whole. On higher doses the sensory input reaches such incredible amounts of depth and intensity that you are entirely overwhelmed and engulfed and they begin to flood out other parts of your consciousness (memory for instance, this is when ego death starts to occur). From a philosophical perspective, you can say you experience a one-ness with the universe because the universe IS one. The universe is one constant process in motion, expressed in various ways. Our brains are attuned to very specific expressions of the universe that only barely reflect what it really is. When all that information and experience is merged back into one phenomenon, we can "recognize" that our perception of who and what we are is a product of this system our brain uses. What we really are as a phenomenon is much bigger than what our senses perceive

Hallucinations occur also because thoughts (like anxiety, or extreme happiness, anything really) are conceptualized by your brain, they are given new depth and dimension and they can manifest themselves as things you experience, as well as being the result of stimulation caused by the glutamate that is released in your brain. So it's not like anything appears in your vision, it's that something you feel has so much dimension to it that your brain gives it a form and shape.

Important things, really important things, will be on the foreground of your experience, they will be enhanced by your brain and a desire to act on your realisations is given to you by amounts of dopamine and norepinephrine that are being released at the same time. People with few problems and worries will often use LSD recreationally, people with deep issues and problems will tend to use LSD therapeutically.

The reason people have bad trips is because these important things manifest themselves and they are not willing to confront or face them. That is the very nature of a bad trip, running away from an overwhelming experience. When you take LSD you need to be aware of everything that CAN happen and be utterly ready to confront them, to surrender yourself to whatever LSD does to you. LSD doesn't create anything, your brain does and LSD simply allows you to. What you feel and experience is not foreign, it is rather the opposite, it's at the very core of your existence because LSD interacts with those parts of your brain that make up who you are, what you are, and what you think. So frightening/intense/strange experiences shouldn't be frightening, they are like a window into the deepest and most important parts of your own mind and they need to be accepted and confronted. With these realisations and this mindset, a bad trip is nearly impossible. (This doesn't count for people with war trauma etc. who have flashbacks, that is another case entirely)

So when you realise all these things and keep them in mind when you take LSD (or even write them down on your come-up) you can almost bend LSD to your will. You can realise that for instance if you play some kaleidoscopes on your screen, some intense music, eat candy, light incense and writhe around on a soft couch or bed that by stimulating all your senses and bombarding them your brain will be stimulated so much you can come to all these connections and realisations easily. When you realise how LSD works you can stimulate deep thought about specific things by stimulating yourself (like looking through your coursebook for college , playing an instrument, drawing, touching and looking at old childhood trinkets, looking at photos of yourself, if you have ancient objects in your house feel them, look at them, etc. etc. etc.)

When so much information is racing through your brain, through all parts of your brain, you gain new perspectives, insights, solutions to problems, as obvious as day. Some people take low doses of LSD and go to concerts, or go to work out, or go to class/work and the entire experience is so enlightening because every single bit of information in those experiences shoots through every part of your brain. Valuable things will hit you at the core of your being and teach you something about what you truly enjoy and value.

The reason you see patterns so much on LSD is not because they suddenly appear, but you learn to recognize them. For instance almost everything in nature grows/is formed according to the golden ratio (http://en.wikipedia.org/wiki/Golden_ratio). LSD helps you spot these patterns, for instance in trees. Not only that, but the inside of your eye is composed of rods and cones that conform to this pattern, which is why you get all these fractals in your vision as well, because you can even experience and observe how this information is received and processed, most people don't even realise it. This is not entirely correct, only partially. The rods and cones conform to a fibonacci pattern, but what we experience as vision is entirely a product of our brain and has, in a very extreme way of speaking, nothing to do with our eyes. What you perceive as vision is what your visual areas produce, and they use information from our eyes to produce it. That information is transformed from the cells that comprise our retina to the cells that comprise our visual system, and THAT transformation is also based on a fibonacci-sequence principle. Think of when you have an image on your screen and you zoom in, the individual pixels change colour to give the idea of zoom, but ultimately it's a 2D surface with fixed points between which the colour-relationships change. In a similar way, your brain converts image from the small surface of your retina, to the much larger surface of your visual area, to give you the experience of sight. Try going to a park and look at some trees as they are silhouetted against the sky, you'll be able to see nothing but fibonacci sequences in which you can literally distinguish the shape of the wind from how every single branch flows individually.

Two nobel-prize winners who both work in biology and natural sciences have credited their discoveries with the realisations they made on LSD. It is no surprise when you realise that LSD interacts with everything in us that makes up... our constituents of life. We exist in the same shape and pattern as everything else in nature and when you are exposed to those fundamentals of life, it is almost easy to conceptualize how it works. THAT is what led to their nobel-prize winning discoveries.

You are able to make equally deep discoveries and realisations about yourself and the world around you because of all of this. Think about that next time you trip.

There is still a host of information missing from this post but I hope I have given you an idea of what LSD does to us and why if you take it your life will change forever.

Edit: jellyculture commented about source citation and that's a great idea, just so you guys know I'm not mumbling utter bullshit here:

LSD Neuroscience, lecture by David Nichols, Ph.D: https://www.youtube.com/watch?v=LbUGRcuA16E

Scientific Problem Solving with Psychedelics, James Fadiman: https://www.youtube.com/watch?v=KtL5fafpRKc

HANDBOOK FOR THE THERAPEUTIC USE OF LYSERGIC ACID DIETHYLAMIDE-25 , 1959: http://www.maps.org/ritesofpassage/lsdhandbook.pdf

The Pharmacology of Lysergic Acid Diethylamide: A Review, Thorsten Passie et al.: http://www.maps.org/research/cluster/psilo-lsd/cns-neuroscience+therapeutics_2008-passie.pdf

Serotonin and Hallucinogens, G.K. Aghajanian, M.D. and G.J. Marek, M.D., Ph.D.: http://staging-www.nature.com/npp/journal/v21/n1s/pdf/1395318a.pdf

Neural correlates of the LSD experience revealed by multimodal neuroimaging, R. L. Carhart-Harris et al, 2016: http://www.pnas.org/content/113/17/4853.full

Edit2: When I wrote this, I had just started up my studies of evolutionary neuroscience. Even though fundamentally everything here is correct (or rather, conforms to what scientists currently suspect about the human brain and how LSD interacts with it) I would have written it differently now that I am actually have a career in evolutionary neuroscience two years later. I've added some things in bold text to avoid confusion and for the sake of clarity and accuracy. I have also added another very important article that was published in the meantime.