I'm really bad for nail biting. I don't just keep them short I tear away strips of nail all the way to the cuticle. Then I tear away at the cuticle untill it bleeds. Then when my nail is far enough back I start biting away at the skin UNDER the nail, which is incredibly painful and again they bleed but I just can't stop.
On the subject of toenails, I used to bite them too when I was younger and had the flexibility.
I think it all stems from the fact that my mum used to cut my nails when I was a toddler and I used to freak out about it, certain she was going to slip and cut me, so I started biting them to ensure they stayed short, after that it's just become a game of "fuck up your nails as much as you can"