The first being a guy who is genuinely kind and caring. He is polite to everyone regardless of sex, age or race. He has no ulterior motive, i.e. he is not nice to get a reward, he behaves as such because it's human decency.
The second kind of nice guy is the one who has ulterior motives. He believes that because he behaves in a certain way the world owes him for his actions. He doesn't make it clear what he desires from the beginning and becomes angry when he doesn't get what he wants.
"So Steve's helping his neighbor move today".
"Really? I hope he gets a reward".
"Oh, he doesn't want one, he's just a nice guy".
"Did you hear? Apparently Matt had a fit when that girl wouldn't go out with him".
"What did he do? Insult her?"
"No, he befriended her and pulled a nice guy act".
"What a dick!"