system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do you smile when you feel happy?

Like, why when we feel happy or good the edges of our lips go up and when we're sad they go down? Why isn't it the other way around?
gpt
Primatologist Signe Preuschoft traces the smile back over 30 million years of evolution to a "fear grin" stemming from monkeys and apes who often used barely clenched teeth to portray to predators that they were harmless, or to signal submission to more dominant group members. 

It's basically a less threatening version of showing your teeth aggressively. A confirmation to others that you are *not* showing your teeth aggressively, because it's gentler.  It shows self-restraint, theoretically, that you're keeping yourself from showing signs of conflict or dominance.