I think hitting on your brothers girlfriend shows you pretty much
still don't care about anyone but yourself, but I'm so not going there this late at night.
The way i see it, Damon takes someone being a normal human being as them actually feeling something for him, because HE is only nice to people when he wants something in return. like Stefan said, he projects his own morals, ideas, paranoias, whatever onto others.
Stefan cares what other people say and think and do, Damon doesn't. (or at least pretends not to. but if he did care, he'd adjust accordingly so people will like him, like Stefan does, and he doesn't )