I think you are confusing nice with non assertive. You can be nice, but you also have to assertive enough to go and ask the girl out or to tell them what you want when it comes down to relationship stuff.
I can imagine a "nice" guy hanging out with a girl, but never really having the guts to say anything to her, even though he really REALLY wants to date her. The girl, probably 90% of the time, will probably be oblivious to his feelings of affection, and will end up with another guy who will be assertive enough to express his feelings and stuff.
So its not the "nice" part that kills the nice guy, its the "self confidence" part that usually screws him over. You can still be an "alpha male" assertive type and also be a nice guy. Those two things aren't mutually exclusive.
