I was thinking a lot about this today and what i think it means, because i am Facebook friends with a guy who has this problem.
Basically to me, it's a guy who is nice to women and then gets mad when women don't reciprocate with a relationship/sex. When the whole reason they're being nice is just to get what they want, women see through the bullshit and they can't take that.
Also it's really important to distinguish the difference between nice guy syndrome and guys that are acting kind. Guys can act kind without any intentions of having a relationship/sex with you, guys with nice guy syndrome act nice to get what they want. When a woman chooses to put a guy with nice guy syndrome in the friend-zone, the nice guy does not sit well with that. A regular guy would get the hint and write it off as a matter of compatibility, which it is. However someone with the nice guy thought process will try to change the woman's mind, because she isn't right, they will stick around and wear the woman down. Nice guy behavior is very manipulative, they will do and say anything to get the end result that they want. Nice guys say things like "women always date douches and i'm nice, why don't they want me?".
The guy i'm friends with on Facebook always says things like "i'm a true romantic, women say they want romance but they really don't. They want guys that treat them like shit. Guess i should treat women like shit then." no- women want romance, they just don't want you. YOU are the problem. Your personality and the way you act is the reason women stay far away from you.
How to fix that? Understand that women owe you nothing, act like yourself and don't lie and manipulate people to get what you want. Don't describe yourself as 'nice'.
Those are my thoughts on it so far, feel free to add to it because i'm sure i missed stuff.