Damon would do anything for Elena, you guys. As long as he believed it was what she wanted. But right now he doesn’t and that’s the problem. Damon doesn’t believe for a second that Elena really wants to basically marry him. He doesn’t believe for a second that she’s going to feel exactly the way she does now when the sire bond is broken. He doesn’t want to invalidate her feelings by saying so when she asks, so he hedges with “I’m saying we don’t know”. But I think in his mind hope that her feelings are real is fading fast.
So if he turns human he won’t be doing it for her, he’ll be doing it for a pipe dream and he’ll have only himself to blame. And what’s worse, he believes that Elena will love Stefan again when the sire bond is broken. And Damon knows his brother. He knows that if she’s human again and loves him again and the two of them are able to pretend nothing ever happened, he will want her again in a heartbeat. And he knows Stefan would take the cure, with or without her, without a second thought. So it’s all too easy for him to see a Stefan/Elena endgame in which the two of them are human and oh so happy and he is human and not happy and the useless part of the group everyone feels guilty about.
I think Shane hit the nail on the head when he said that Damon felt his relevance slipping away. Because I think that’s exactly what he fears – being irrelevant. And that’s what human beings are, as a general rule. They’re irrelevant except in the measure in which somebody loves them, somebody wants them, somebody needs them, somebody calls them their own. And I don’t think Damon thinks he can have that with Elena or with anybody. So what would be the point of him as a human, exactly?
In Damon’s mind I think it’s better to stay a vampire so he can at least mean that to Elena and Stefan even as they mean everything to each other. Because as a vampire he can protect them where they can no longer protect themselves. As a vampire he can save them from the nasty little happenstances of human life, like car wrecks and cancers and sick children and even unbearable sadness. He can still be the hero if he can’t be anything else. And hey, he can still be the eternal stud, and he wasn’t lying when he said he liked that." — Rophydoes (x)