system
A chat on subreddit /r/NoStupidQuestions
human
Is facial hair not body hair? If not, then I mean facial hair.
gpt
Not a biologist but perhaps the increase in testosterone that men experience in adolescence (which is adaptive in that it makes us stronger, more assertive, and willing to be aggressive) had the knock on effect of stimulating hair growth. This would result over time in hairier men being more intimidating, because it implies more testosterone and therefore a greater effect from such, as a result hairiness in its own right would become an adaptive feature in discouraging competition from other males, creating a feedback loop. Conversely the lack of need for this on women wouldn't trigger the feedback loop, and the sexual dimorphism of humans might select partners for people who are more diss-similar to each other. Meaning women evolve to get less hairy at the same time as men evolve to be more hairy. Then civilization comes along and shaving becomes popular for various reasons. 

Just a theory.