Tyler can't pinpoint the moment it started; all he knows now is that he has this undeniable urge to possess Elena Gilbert.
He never felt it when she was with Matt—at least, he didn't think that's what it was. He remembers feeling envious of her because she started to get half of Matt's attention when he used to get all of Matt's attention. He used to wonder what it would be like if he hadn't convinced Matt that talking to her was a good idea. But even at a young age, he had known that there was no going back, and Elena Gilbert was going to forever be a part of his life through Matt Donavan.
Forever ended after Elena's parents died, however. It was unexpected and shocking, and it was the first death-of-a-parent that any of their generation of Mystic Falls children had to experience. Tyler remembers the way Elena wouldn't stop crying, remembers the way Matt had tried to comfort her in any way he could, but couldn't find the words.
Time passed and with the arrival of Stefan Salvatore and his brother, Tyler's Elena affliction worsened. At the time, he thought he was just angry on behalf of Matt, who couldn't be angry himself—but now…he's not so sure.
Those Salvatores shouldn't be allowed to waltz into town and steal the girl he'd known since they were practically in diapers. It's not right. But then there's Mason and werewolves and Caroline and Jules and so much happens that he forgets about Elena for a moment because he has to remember to deal with himself.
And Jules and Brady are telling him there's a way to stop the curse, to be able to turn at will or not at all, and they have to get this moonstone and eradicate the vampires and all sorts of crazy shit and he listens because he needs someone to tell him the truth and he thinks they're it. But Stefan's telling him he'd have to kill Elena and he knows he can't do that.
Because it's Elena and he needs her to be around and so he lets Stefan go and Brady is killed and he doesn't feel guilty because Elena gives him this hug and he so desperately clings to it.
He leaves with Jules after that because he doesn't know how to be around Elena when he feels guilty for almost getting her killed. He doesn't know how to be around her when he looks at her and all he wants to do is push her against a wall and kiss her.
Later, he asks Jules to shut it out for him and she laughs. "It's your madness, Tyler," she lectures. "You have to figure out what you're going to do about it."