If I knew why I was asexual, I'd tell you. But the truth is, little is known about sexual orientation as a whole, so I don't any more know why some people are asexual like me than I know why some people are gay/straight/bi/pansexual/whatevers. I guess there are a lot of asexuals in fandom because fandom has a lot of everyone.

But hey, even if she isn't romantically available, just be happy you have such a great friend in your life, and cherish one another and the closeness you have. Whether it comes in the form of platonic or romantic love, what you have is rare and special, and you should treasure every moment you have together.