Why do some people progress faster than others?

[akai]

Looking back I think I started thinning at 18, but the progression was so slow (crown and some diffuse thinning) that I didn't really notice until 23/24. Recently saw a friend over the holiday who now razors his head because he's in NW4 territory. The thing is the last time I saw him a year and a half ago he basically had a full head of hair. How can someone make it to their late 20s with little or no hair loss and then BAM most of it's gone in 14 months?