There's a difference between being a nice guy, being a
Nice Guy and being a fucking pussy.
Sadly, a lot of guys who aren't exactly, shall we say, "alpha males" mistake being nice for being a doormat and then complain when people walk all over them.
Sadly, a lot of guys who call themselves "nice" are really just incredibly passive-aggressive.
I have yet to break up with someone for being "too nice" but when he (or she, actually) answers "whatever you want, dear" to everything I ask I have a hard time respecting him. I don't like jerks either, trust me, but I want my partner to be their own person, and I don't expect them to accomodate me with everything.