Female takes away her humanity and reduces her to an object.
Men like this have only ever experienced serious relationships from an outside perspective looking in, or from books, movies... and so their entire view on what a relationship is and should be is incomplete, distorted, and shallow.
They have no idea what any woman (n. female) actually wants from a relationship, but their immature entitled world view—i.e., that they deserve a relationship, simply for being a man, who exists—means it doesn't matter. And so when he desires a woman, he sets out to get one in the same way you might set out to get a pet.
The problem quickly becomes apparent when he makes an advance of some sort. At this point, the Nice Guy will suffer (a usually polite) rejection.
For a normal person this would be brushed off and forgotten.
For the Nice Guy this is just confusing.
He did everything right. He even opened a door for her, complimented her, or perhaps made some awkward and inappropriately grand romantic gesture.
Why then didn't she accept his advances? Everything the Nice Guy did was, well, nice!
And because the Nice Guy approaches romance from a black and white perspective, someone must be objectively wrong in this situation.
Unfortunately he lacks the respect for the woman and the social awareness necessary to seriously consider himself to have been at fault...
So the woman is wrong.
Perhaps she's too stupid to see the good in him, perhaps she chooses to date guys who don't fit the perfect "gentleman" archetype: the Assholes and Douchebags who don't understand how to be nice like he does. The guys who will most likely beat and cheat!
This women then is not a Proper Lady™ like he had probably thought. She's reduced to a Whore, a Bad Bitch, or some other object.
All that really matters is that she's wrong.
Ego protected.
Self-improvement is then out of the question, but fortunately there are other women, and surely they will understand the rules of courting! Some of them must surely be Proper Ladies—and they will want a gentleman like himself!
This scenario repeats itself many times, until he eventually he realizes that most females are Whores and Sluts who only want Douchebags and Assholes, and he'll probably never find his Proper Lady.
He'll forever lack the maturity and self awareness to change his behavior. And why should he?
He's a nice guy.