Why do so many girls think "being one of the guys" is an insult?

I'm noticing this a lot! I think it's one of the best complements. It means you're a friend that I'm comfortable enough with that I don't really see that I have to give you special treatment because you're a girl. Of course, I do different things for them! Like, opening doors and paying for stuff. But why do so many girls feel offended rather than complemented?