Why do we bite our nails?

Those of us that do obviously… I know it’s probably a nervous thing for many or stress… but WHY? What is it about biting nails which is so…stress relieving(because to my mind it is not in fact very stress relieving – it can be rather painful sometimes!)

I also sometimes just quite like the feel of biting ym nails which is a little odd

so any thoughts…would appreciate them