Hi there, Former Nice Guy Stereotype here with some chat words straight from my fingertips to your eyeballs. Now I'm aware that these sorts of things can happen to anybody of any gender, but since the more common examples are men pursuing women, I'll mostly be using pronouns to that effect.
Now I do still consider myself a nice person (as others have said though, don't go around CLAIMING to be a nice person, and don't let that be your defining quality) but here's the thing...being nice means being nice to everyone. Not just women you want to date and probably some day sleep with, and it also means staying nice in the event that she says No. Or at the very least not treating her like she has somehow committed some great offense by denying your advances.
However, being nice also doesn't mean you're a freakin' saint, as some people claim it does. If you like a girl, and she wants to just be friends, and it drives you crazy being around her...leave. That doesn't make you a jerk. That makes you a human being trying to deal with their issues and emotions. Any girl who would try to force you to go through emotional anguish like that or try to guilt you into staying around is probably someone you didn't wanna get involved with anyway as they are clearly quite selfish. They may tell you that if you truly were her friend you'd be able to stick around "just because I don't wanna sleep with you." (as if that was the the ONLY thing) but if she was truly your friend, she'd be able to understand why that might be tough. Sometimes it can be done, men and women can stay friends even after rejection has happened, but sometimes it is really hard, and the best thing for both parties is to just move on.
Now onto the "friend zone" thing. Now here's the thing about the friend zone...it exists. It does. Anyone who says it doesn't is just plain wrong. HOWEVER what the friend zone is and what it means has been so warped into a widely spread (but incorrect) definition that it's easy to see why people might think it's just something guys make up.
See, when most people hear the term "friend zone" they think "Oh, a so called nice guy is whining because a girl he wanted to bang turned him down, so now he's using that as an imaginary shield to explain his problem rather than blahbity blah blah blah." That's just wrong. This is what the friend zone actually is, and since it can happen to anyone, I'll avoid using gender specific pronouns
Person A meets Person B. Maybe Person A was already at least somewhat interested in Person B when they met, maybe feelings developed throughout the course of their friendship, it doesn't really matter. What does matter is that now, Person A has romantic feelings for Person B. Person A informs Person B of those feelings, but Person B just doesn't feel that way about them but would like to stay friends.
That's it. That's what the friend zone is. It's simply when someone you have romantic feelings towards does not share those feelings but would like to stay friends. That is as far as it extends.
The biggest problem with the stereotypical nice guys is that they read that scenario and probably think Person B is a total bitchdick. Which isn't true. Someone not returning your feelings doesn't make them a bad person. The sooner people realize this, the better off we'll all be.
Now I'll confess that since abandoning a lot of the negative nice guy stereotypical traits, I haven't exactly been any more successful with women than I was before. I may be more confident, direct, open, and mature...but I'm still not exactly super attractive, I'm still pretty weird, and I still have a lot of self-esteem issues and self-loathing that probably show a lot more than I think they do, so I still get turned down a lot.
But here's the thing...that's okay. Once you stop treating every rejection as some sort of personal attack on your character and realize that "Hey, maybe I'm just not that person's type. Let's move on." dating becomes a lot easier. Focus on being yourself, rather than being the person you think she should be dating, and don't act like they're just in the wrong for not wanting to date you.
This post ended up being a lot longer than I expected it to, but I hope it's helped someone. If I've helped at least one person, I've done my job.