Guh. Okay. Being a girl who has broken up with a "too nice" guy before, I can translate this one from Girleese to real human English. "Too Nice" is like "Nice Guy". Nobody is ever a Nice Guy. He's a passive aggressive dick who sits by and eyes you while he "waits" and watches the girl pick the right guy. She knows he wants her, she also knows he's not going to step up to the plate and she resents his passive aggressive behavior.
She broke up with you because you were passive and boring. Now, you may not be passive and boring to all women, but to her you were. She was bored and she wanted out and she couldn't think of anything else to tell you. Maybe you guys didn't go anywhere interesting together, maybe she wasn't into the sex, maybe you agreed with her all the time, maybe she just wanted a little conflict, a little adventure. The guy I left for being "too nice" I didn't understand why I was unhappy (I was only 19) because there was nothing "wrong", I just hated it and needed to get out. We only sat around at his house and watched TV (which was perfectly nice, but boring). We never went anywhere, the sex was uninspired (but nice), when we began to argue he'd cave to everything I wanted (which was the "nice" thing to do, but way too passive). The relationship wasn't two people coming together. It was me, and a half formed human being who didn't have his own interests, dreams, or opinions.
Now, I can only speak for myself and the few friends I know who have left "too nice" people, but it's never because someone's too nice, it's because the person is too passive. If you know that you're not a passive guy, then pass on her! She's clearly got issues if she feels that she needs to be with someone who treats her poorly. If you continue getting this response and you know you're not a human doormat, you're dating the wrong people.