My beard grew when I stopped shaving the bottom end of my face, many years ago, because I was always in a rush in the mornings and didn't fancy looking myself in the face at that time of the day.

Noone goes asking why someone is so eccentric as not to shave the top of their head. What's the difference?

A few years ago I got curious what I looked like under that, and shaved it off. My wife took one look and said "grow it again immediately."