I don't grow armpit hair?

I'm 17 years old. I've already gone through puberty for the most part, if not entirely. I'm a dark brunette (naturally) so my leg hair, pubic, arm and facial hair comes in super dark and super fast. I've been shaving my legs since I was 13. But I have NEVER grown armpit hair. I have a habit of checking every time I shower and never see or feel anything. It's always crazy smooth. I've even shaved my armpits just to see if they shaved any hair off and they didn't. Should I be concerned? Is this normal? Do any other girls deal with this? My best friend is a natural light brunette and gets in crazy amounts of dark armpit hair if she doesn't shave. I just don't know how common this is. Thanks!