it's not that you're nice, it's that you convey yourself as needy and wimpish - two former traits which are commonly associated with being nice - it's the wrong brand of "niceness"
you need to be a nice person who is tough, if that makes sense?
you need to be humble but you also need to be confident - confident enough to be cheeky and bold
that is the key to being nice + romantically successful; if you're nice and sweet but you can make her chuckle and give a bit of push-pull then you've got it (Y)
I used to try the bad boy act, and it worked sometimes (that's better than "never"] with some girls (the girls that would probably like the kind of alt. hipsterish guy that I am) but what worked recently was to simply be myself and be nice to the point where I wasn't being needy - I was a bit menacing with my approach (teasing her and being playful whenever I could) but still warm and nice enough to be a decent person, e.g. listening to her, and actually caring about what she had to say, and even saying "thank you"/"please" appropriately (if that's not too obvious...). a girl wants to have fun with a guy, and if literally the only thing that you can be is "nice" and not "funny and interesting" then you've got nothing...anybody can be nice - it takes somebody special to be more) to press these other buttons...I was also not being "nice" to the point where, instead of asking permission to do things, I was making commands, e.g. "come with me", "don't do that", etc - that displays at least *some* confidence if I'm willing to take control, e.g. like a man, right? right! so that's what you've got to convey - a nice but manly image/personality
also, don't equate "manliness" with being physically aggressive to other men for her sake - that's stupid and would only attract an absolute slapper...and it has got me into fights that I didn't end up winning. because other guys are sometimes willing to fight other guys for girls, even if they don't know those girls...I've learned from those mistakes now. I even had a guy trying to literally steal a girl I was bringing home with me (a housemate, actually) and I was refusing to fight him when he was trying to snatch her (not that she's an object or something) he was saying "what are you going to do about it?" - in the end, the girl actually went off with me *because* I wasn't going to fight him. that also probably shows that I wasn't needy.