The question is not "Why am I a girl?" but "Why hadn't I thought about this sooner?"
95% of the boys who act this question is because they think a perfectly natural thing of playing as the opposite gender would be something others would look down upon, or they will get flamed, and shut the thought that they think it might be cool, or they're curious, or some other such notion, and automatically deem it as bad so they ask is so their so called "masculinity" is not hurt and their fellow players think the person is asking to avoid people thinking he wanted to play as a girl.
What you should do, is accept that you're playing the role of a girl, and live with it. It's not like you are a girl because of this, and no one will flame a boy playing as a girl in an online game.
