One of my college roommates was like that for a while. Then, fortunately, she met a guy she liked with whom she could also be friends, so that relationship worked much better. In fact, they got married last year. So there is hope, I think. :-)

However, I'm having a repeat of that experience with one of my coworkers, M., who's a great person but has already, this year, gone through at least four romantic relationships, one of which was a kind of unofficial engagement. (I knew that wasn't going to last, but didn't say anything until it fell apart on its own and she was willing to listen.) It makes me sad, because she's always getting hurt when things fall apart, but I don't know any way to point out that she's probably approaching her problems from the wrong direction. *sigh*