I think "know it all"s answer is pretty good. The eternally-single, self-proclaimed nice-guy as a fair amount of denial going on. Not in that they're a good or bad person. We're all pretty much the same at our cores. But denial that they may have anything to do with their situation. They're basically claiming to be the victim of being too wonderful, which is nonsense. As many books are written on the subject of relationships, as many couples don't make it, as many couples counselors there are, they're basically claiming they're approach to the opposite sex is perfect, needs no modifying or tweaking. So naturally the problem must be with the opposite sex. Perfection always requires a scapegoat.
IMO, romantic fantasy addiction and perhaps even sexual addiction are a significant part of the problem, at lease with the guys in this discussion. Addictions typically produce loss if interest in normal activities(e.g. normal relationships) and social anxiety. Many of these guys will claim no girl is interested in them, while conveniently forgetting the girls that actually were interested but seemed dull and boring compared with that just-out-of-reach girl over there who bounces from guy to guy but will surely one day see the light and get head-over-heels in love with him. THIS girl seems to have the magic, and he spends endless hours wishing, hoping, dreaming and hating. Romantic fantasy has taken precedence over real relationships with real females. But of course, if he could just get one of the "good ones" there'd be no more need for fantasy, wishing or hoping. And this thinking is not much different than an alcoholic thinking if he/she just had an endless supply of booze and didn't have those pesky realities like a job, a spouse and friends standing in the way of them and being able to truly enjoy their drinking, all would be well with the world.
The female version of this might be that girl who reads romance novel after romance novel, waiting for that hunky guy who's a CEO or internationally famous cat-burglar to come into her life and sweep her off her feet. And all this will happen when she's lying on the couch in her one-piece hooded pajamas watching soap operas. He'll just burst through the window or something like that.