I actually used to be one of these "nice guys". I was never a popular jock, and I had a lot of self confidence issues coming out of high school and going into college. Time and time again, girls I would date would dump me for much cooler guys. I was too intense. I was easy to develop STRONG feelings for a girl after one date or something. Eventually I became so bitter with all women and the institution of dating that I told myself I didn't care anymore.
So I went an entire year without attempting to date anybody. I hung out with my friends and just enjoyed college life. Then eventually, I truly just didn't care. I was having a great time, figuring out myself and where I fit in, and was gaining a lot of self confidence. I started being a nice person just because I enjoyed being nice to people, and I didn't care what they thought of me for it. Eventually I met somebody that made me want to date again, and this time it was much easier. I didn't put her on a pedastal, we were just two equals who weren't playing any games, we just liked being together. Now we've been together 5 years, married a year and a half, and we're pretty happy.
The point is, once you do have some grasp on the concept of self esteem, it makes it easier for you to interact with the opposite sex, and it makes them more easily attracted to you.