Why girls don't want nice guys?

Maybe i should first tell "my" definition of nice, because i have saw a lot of girls think of it as needy or for that matter i should describe myself a little bit. I am kind of guy who will always support girl and always be there for her no matter what the problem is. I am always looking to please her and i never care what others tell about her because i have my own opinions and because of fact i am in relationship with her i should trust her more than friends. When we are speaking of friends there is one more thing i would like to know why girls care so much about their friends opinions. I mean it costed me one relationship. I was telling her when we started(which i always do) that her friends are gonna be jealous of us. So they were they kept feeding her with talk like i am not for her i am just trying to use her etc. and then we broke up. For example my brother is every night with other girl and I am sick of it I mean i can't stand that why girls are acting so low i mean even thinking about it hurts maybe i am too emotional when it comes to females :P. It doesn't matter if it is my brother or someone else its just i could never do that one night stand or whatever. I always try to stay friend with my ex-girlfriend all through there is not a lot of them but nvm that now. I am saying that because they and relationship with me because its "too easy" they are like they don't need anymore to try hard to keep me or something like that and then they go to someone other. After a while they broke up and girls come to me crying how he just used here and how there are no more nice guys. Then my reaction is like WTF? I am really trying to figure out what girls want I will get over anything if i love her just not cheating. Basically from my experiences i should just play cold hearted guy who doesn't care about anything, show no emotions or interest in girl and that relationship will last? I hope there will be answers from girls. Sorry for mistakes English is my 2nd language.