why the complications?

Grey's anatomy! Best series ever. to me. And it seems so realistic that at times, when shit happens in that movie in terms of deaths or couples breaking up and hooking up with another colleague, it sorta upsets me. But the one thing that's practically lingering in my mind after s06, episode 10, the scene where little grey (Lexie) broke up with mark sloan and right after that, Lexie starts making out with Alex. Why is relationship so insignificant to them? I mean, when one is over, shouldn't they be revolted at the thought of mere making out with another person. Especially if that someone is one of your friends? Okay, so i've heard of rebound and whatnot. But i still don't get the idea of anyone actually doing that?? Sigh. Someone please please tell me why are such things even happening?