I am gonna go out on a limb and say you probably act a little desperate. Your reaction gives the impression you want this girl (or girls generally) too much and when you don't get them, it hurts and frustrates you.
Also, maybe you are rushing things (for fear of losing your opportunity perhaps?) I blame the community/gurus for preaching this urgent mindset, 3 second rules and all the rest of it that makes guys seem desperate. Take your time, build rapport and comfort, and NEVER pressure a woman. I've held back on some women for weeks to months, enjoyed being friends and in that time their feelings have grown and they've fallen in love with me - before I even dated them.
Metalfortress has a point too, that maybe you're being "nice" because you have ulterior motives. If you were truly nice, you'd accept being friends because it's what she wants. If she senses ulterior motives to you being nice, then it's game over cos it's basically ass kissing and supplication.
It's easy to say "don't be so desperate" or "stop caring so much", but it's not easy to put into practice instantly. I was in the same situation for ages and couldn't understand why I kept getting rejected, but as soon as I gave women less priority in my life, the more successful I become.
As a lot of guys point out on this board, try seeking more distractions and positive things in life that don't involve girls. I'm at the stage now where I'll actually postpone dates with my girlfriend if I have university essays to write or if I've already made plans with friends. I love being with her and it goes against my instincts; it even makes her think I don't care sometimes, but her interest is sky high.
I feel your frustration as I've been there many many times before, but it's a bad mindset to pursue women. And ironically, guys are usually most successful with women when they're not looking or already have a girlfriend.
Good luck to you anyway.