Background
Nice guys don't get many dates, because they feel more comfortable going out with a woman who already expresses an interest in them. There's any number of nice guys in the world, and most of them are content to be that way. However, some nice guys develop a psychological defense mechanism about their lack of dates, and this is the beginning of the nice guy complex.
Diagnosis
A man suffering the nice guy complex begins by looking around and seeing everyone else going out with more women than he is. (Often they get this idea from television rather than their friends.) He concludes that this is only because he's a nice guy, and he doesn't prey on women aggressively like everyone else obviously does. After all, doesn't everyone know nice guys finish last?
In reality, he may not be expressing his interests in those women to whom he's attracted, or he may be so focused on one particular woman that he refuses to consider other women at all. Perhaps he's not really the nice guy he thinks he is, and all the other women are put off by the personality they see in him. Or perhaps he's simply dull and boring. ("No way, not me!")
Dangers
It's this gradual obsession with the guy's own niceness being the reason he's unattached that creates the nice guy complex. This may become harmful, if the nice guy allows his ego to become inflated over how nice he really is, thus making him "not-so-nice-after-all".
Whether it does or doesn't, though, it's generally crippling to the guy's social life. The guy believes he has no redeeming qualities that would attract women, other than his own niceness. Eventually he may stop trying to be attractive at all, either in looks, interests, or attitude, thinking that niceness ought to be enough, and "Shouldn't a woman love me just for who I am?"
Treatment
The best treatment for the nice guy complex is self-improvement. An exercise program, art classes, dance lessons, or learning to cook are all great beginnings. The idea is to develop something that will help the guy be proud of himself, so that he doesn't have to dwell on his niceness as his primary redeeming feature. The idea is to progress from being merely a nice guy into an actual gentleman.
The irony, of course, is that developing any of these talents may attract a woman to him who will tell him, some day, that the thing she always loved about him is what a nice guy he is. Guys, don't believe her if this happens. All women love nice guys, but that's the last thing she really learns about you; what impressed her is the way you could dance or draw or, possibly, the way your butt moves when you jog.