Nice guy syndrome basically describes a guy who will listen to his female friend's every complaint, rant, problem, and experience in life; will help them out no matter to the cost to them; would die for their friends without even thinking; but can never get a girlfriend.
All their female friends will continue to go out with mindless, stupid men who treat them badly, and hurt them over and over again, never even considering the nice guy, because he is “such a good friend”.