The "Nice Guy"---and Why He's Not Nice
"Nice," as used in the community, is way different than "nice" as used by women.
When the community uses the term, it is a perjorative. But since to most people it's enjoys a positive connotation, I thought a little more definition would be helpful
The "nice" guy likes a girl. He starts by befriending the girl and denying any romantic interest (he figures: I'll prove I like her as a person, not just a romantic interest), secretly taking note of what she likes, and then giving the "perfect" gift along with a confession of his love.
Poor girl! She thought she had a friend, but now she finds out he was just acting the whole time. Alternatively, she is uncomfortably surprised from this lightning bolt out of a seemingly clear blue sky, and is worried about how to save the friendship without hooking up with the guy, perhaps adding to the rejection the caveat, "You're such a nice guy."
Eventually, she lets him down as easy as she knows how (remember, this is a good, kind girl here), and this guy wonders what he did wrong. He certainly didn't do anything mean; his ears perked up when he heard her complain about guys, and steadfastly worked to avoid doing those things. He figures he's being nice.
The problem is that these guys have a sort of male version of going for committment too fast. A relationship is a 50-50 venture, but he's gone 90-10, and he's basically trying to pressure her into liking him, claiming (or at least implying) that she "owes" him for all he's done for her. He is trying to force her into liking him, she senses the trap, and she splits.
Then she meets another guy. She regards him as moderately attractive (maybe even LESS attractive than the nice guy), and she figures he'd be worth a date. So they go out, and it's clear that they're on the same page---he had an open Friday night, so, hey, he called her up. Turns out he has a rough edge or two, but nobody has to be perfect. He calls her up for a second date, and again, it's clear to both that this is just a date. She hasn't put that much effort into it, but neither has he. At least he isn't trying to lasso her with guilt.
So when our self-proclaimed "nice" guy meets this new guy and sees that he's not working as hard as the "nice" guy did, yet she's dating HIM, he thinks: "What? I worked so hard for her! I did everything I could! I even watched her favorite show so I could find something to talk about! What's wrong? Why is she dating that jerk?"
The so-called "jerk" isn't really a jerk; he simply isn't as fawning as the "nice" guy. But the labels have been assigned in his head, and so comes the saying, "Nice guys finish last."