The thing with friendships of the opposite sex is that usually unless it's a superficial friendship you're going to share your problems, flaws, feelings, being affectionate etc. Because you're not looking for a relationship you usually accept them unconditionally. In that environment one person is going to fall for the other. It's almost inevitable.
Exactly how I ended up with my first wife. We were neighbors in college, became good friends, I fell for her (because that's what I do) and she decided I was a good catch and it was ok to condemn me to a life with no passion.
I can't blame her though; we were both kids. Didn't yet know how the world worked.