He doesn't truly know when the hate and unhappiness started to fester in him. He remembers a time when he was happy, ecstatic even and it is that happiness that makes him feel its absence so acutely but the in between is lost to him.
Perhaps it was the first 'no' that Odin told him that he would not tell Thor. There had been many of those.
Perhaps it was the first secret his mother had whispered in his ears that had made him crave more. There had been many of those.
Perhaps it was the many slights that Thor had played off as jokes, ignoring that Loki was laughed at and not with. There had been many of those.
'You were both born to be kings'
Only they weren't. How could Odin begrudge him his lying nature when he had been the very first to lie to him? When his very existence was but a lie what else would he be the God of?
'I could have done it father! I could have done it! For you, for all of us!'
Only not really. He hadn't done it for anyone but himself. He had to be the hero or he would be the monster and monsters are slayed. He tried to be the hero again by slaying himself, the monster that he was. But that was not allowed and instead he was saved to be the villain in yet another story. Never mind then, this was his fate. To be the one who laid waste to lands, to be tear in the eyes of innocents.
He would play the monster to the hilt and wait for the hero to slay him. If he could not be remembered in his own glory then he would be remembered in another's.