Girls say that they want a nice guy because they associate the "a**holes" with cheating and getting hurt. They go for the bad boys because they are confident. The saying, "nice guys finish last" has some truth to it. When a guy is "too nice" it translates to a girl that the guy is "too easy." Both men and women operate like this (girls, a must read is "Why Men Marry Bitches" its a book about why men are turned off by girls that are too nice.)
Anyways, this is how you should think about it. Getting a girl's attention is a lot like playing with a cat and cat toy. If you are teasing the cat letting them catch the toy for a second then pulling it away quickly the cat is interested and excited for the chase. But then once you let the cat have the toy fully and you are not pulling in away, they quickly get bored with the toy and walk away to find something else to play with.
This is human nature. We want what we can't have. Everyone is like this, I don't care how much you disagree and say "Im not like that at all, I don't play games." Yes you do. If you got too comfortable in a relationship where you had everything you have ever wanted from a person for too long, it would be a matter of time before you would feel like something is missing and you would get bored. This is how the human mind operates.
Dolly Parton was once asked, "what makes a successful relationship?" she said, "its give and take. As soon as I can tell that my hubbie is getting too comfortable with me I will pull back and ruffle his feathers a bit."
Bottom line, the best things in life are the things you have to WORK to get. The things that you need to dedicate time and effort into and that applies to relationships to. Women fall in love with men that challenge them not men that hand them whatever you want.
All this doesn't mean you can't "be yourself" or "let your feelings go" the key is time. Loosen up to the girl slowly. Give her what she wants over time. Even if all you want to do is give her what she is looking for don't do it. In the end, the resistance and fire under her butt is what she was looking for the whole time. If things get too comfortable with a girl then pull back. It doesn't have to be dramatic, just enough so that she is reminded once in a while what a catch you really are.
Being too nice means a girl gets whatever she wants without trying and when that happens the girl doesn't value what she receives. When you have to work for it you value it more. Girls date guys that don't let them have whatever they want without a little effort. So what you see is a girl dating an a**hole but really its a girl dating a guy that won't let her walk all over him and give her wants she wants without a little effort. Not giving a girl want she wants may seem like an a**hole move to you, but really it's what the girls wants and what she will stick around for.