Why Myrtle or anyone becomes a ghost

I just remember seeing a pair of great big yellow eyes. My whole body sort of seized up, and then I was floating away...' She looked dreamily at Harry. 'And then I cam back again. I was determined to haunt Olive Hornby, you see. Oh, she was sorry she'd ever laughed at my glasses.'

THE CHAMBER OF SECRETS - Pg 316

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'Oh, yes, very easy to forget Myrtle's dead,' said Myrtle, gulping, looking at him out of swollen eyes. 'Nobody missed me, even when I was alive. Took them hours and hours to find my body - I know, I was sitting there waiting for them. Olive Hornby came into the bathroom - "Are you in here again, sulking, Myrtle?" she said. "Because Professor Dippet asked me to look for you -" And then she saw my body... ooooh, she didn't forget it until her dying day, I made sure of that... followed her around and reminded her, I did, I remember at her brother's wedding -'
'- and then, of course, she went to the Ministry of Magic to stop me stalking her, so I had to come back here and live in my toilet.'

Harry Potter and the Goblet of Fire, THE EGG AND THE EYE - Pg 192
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'But you did!' said Harry angrily. 'You came back — you're dead and you didn't disappear — '

'Wizards can leave an imprint of themselves upon the earth, to walk palely where their living selves once trod,' said Nick miser-ably. 'But very few wizards choose that path.’
.
'He will not come back,' repeated Nick. 'He will have . . . gone on.'

'What d'you mean, "gone on"?' said Harry quickly. 'Gone on where? Listen — what happens when you die, anyway? Where do you go? Why doesn't everyone come back? Why isn't this place full of ghosts? Why — ?'

'I cannot answer,' said Nick.

'You're dead, aren't you?' said Harry exasperatedly. 'Who can answer better than you?'

'I was afraid of death,' said Nick softly. 'I chose to remain behind. I sometimes wonder whether I oughtn't to have . . . well, that is neither here nor there . . . in fact, I am neither here nor there . . .' He gave a small sad chuckle. 'I know nothing of the secrets of death, Harry, for I chose my feeble imitation of life instead. I believe learned wizards study the matter in the Department of Mysteries — '

'Don't talk to me about that place!' said Harry fiercely.

'I am sorry not to have been more help,' said Nick gently. 'Well . . . well, do excuse me . . . the feast, you know...'

The Order of Phoenix | The Second War