My five:
1) Letting themselves get "comfortable" in a relationship, either with relationship maintenance, or personal maintenance.
2) Getting involved to cure their loneliness or to get a shot at motherhood before their biological clock goes off
3) Picking a man based on how he makes her feel instead of how he treats her (I know this goes against nature, but I might as well be honest that nature kinda buggers things up)
4) Controlling the relationship
5) Letting "baggage" pile up by avoiding self-improvement
Now, onto your friend....
He said that he had a girl who turned him down for dates ove and over again and then wanted to be really involed in his life on a highly personal level.
The problem is he's too damn desperate to fulfill some kind of "need" (probably happiness) that he believes the woman can provide. The hard cold truth is, the woman is only "needed" to reproduce (the same can be said for man). Happiness should come from one's self, not from someone else. If you friend keeps trying to build his life on the existance of a woman, his life is going to crumble if she should decide to leave him. He should be building his life on the existance of himself and his own happiness. That's when this woman won't matter to him.
Also, he should work toward dating women before deciding he would like to get involved in a serious relationship with them.