There is no possible way to explain why Damon Salvatore was the way he was.
Perhaps it was how his father treated him. Well, not how he treated him exactly, but more how he treated Stefan. As if little Stefanie was the epitome of all goodness in the world. Damon would try his hardest in school, always studying to try and earn at least a satisfactory grade on a test, while Stefan came home and played all night long, and got a perfect score the next day. It pained Damon that when Stefan always broke the vase, their father always assumed it was him. Damon always took the blame. He had to at least love one of his sons, right? But Damon never held that against Stefan.
Maybe it was Katherine. His first love, the woman who ended and began his life at the same time. He believed that he would ask her to marry him one day, because he honestly believed no one could make him feel the way she did. She was downright gorgeous, and she chose him. It amazed Damon how much he loved her, even after she turned him. He was pissed that humanity was no longer his, but hey, if he got to spend forever with Katherine, how could he stay mad for long? He was in bliss, until he found out Stefan and Katherine loved each other, and Katherine happened to love Stefan a bit more.
See a pattern?
He loved Stefan, he really did. It was just so much fun to torture him. After all, he had taken his father's and Katherine's love away from his grasp.
Payback has been known to be quite the bitch.
Maybe it was the fact that his little brother kept taking everything he worked for away from him, without even trying. He supposes it was why he enjoyed being with Caroline so much. Sure, the girl was whiny and pathetic and she irritated him beyond belief, but she was his, even if for only a moment. Stefan didn't jump at the chance to take her away, and he was going to relish in the feeling that something was only his for as long as possible, but then the whole trying-to-kill-her-but-ending-up-with-vervain-in-your-system incident happened blah blah blah...
Damn Stefan! Damn him and his forehead-brooding, Thumper-eating, tortured-soul ways!!!
Now to Elena, a.k.a. Katherine 2.0. Well, being she was a carbon copy of his love, Damon couldn't deny she was beautiful, but she was so different. Her eyes looked at you with compassion and not lust, her laughs were carefree and not seductive, her smiles were as alluring as Katherine's but Elena didn't seem to be trying to make them that way. And, as with everything, Stefan was in the way. Stefan and Elena were already going through the coupley lovey-dovey complicated crap by the time he met her, so she was off-limits.
Off-limits my ass.
It's not fair. Katherine chose Stefan, so Elena should choose him. It would restore balance to the universe. He didn't love Elena yet, that much he knew, but he couldn't deny that he cared about the seventeen-year old human girl. She was innocent and loving, and he was sure he was corrupting the perfection that is Elena Gilbert with his human-feeding vampire ways, but he knew she felt drawn to him, and he was going to use that to his full advantage.
He didn't know whether he'd ever get over Katherine.
He didn't know whether the relationship between him and Stefan would ever be the same.
He didn't know if he'd ever be happy.
But one thing he did know is that Stefan will not have Elena.
Not if the eldest Salvatore had anything to say about it.