Stiles was a bit of an ass in this one, but I guess I understand it?
Derek pushes Stiles away HARD, for his own good, so that Stiles goes away to college. Now, Stiles is back, nabbed by Deucalion, and pissed at Derek, still.
Derek finally admits why he pushed Stiles away, tells him he always liked him, and then the sex happens, after which Stiles runs off.
Like I said, I get why Stiles was so angry at Derek, but it still felt like he was just TRYING to hurt the guy. I can see it though, I can see Stiles being this way.