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

I don't mean emotional side, because that's all I can really find at google.

From what I understand all other mammals show their teeth in aggression. We know what we mean by it, we smile to the babies and show them it's a good sign when someone smiles, but when we figured that out?  When humans started smiling to each other to signal good emotions? Why did they use this signal, when others use it as a warning and aggression?
gpt
Herbivores do not show their teeth as a sign of aggression.  It is a predator thing.  And humans are omnivores, not predators.  So showing our teeth was probably not an aggression thing.

Humans have much more complex communication than other animals.  We blush to show shame, smile to show happiness, frown to show displeasure, etc.

Humans are incredibly social and evolved to display their emotions clearly to their group.  When you need to display a lot of different emotions you just use every option you can, and smiling was one option.

Basically every way you can shape your mouth communicates some emotion or idea.  Likewise many ways you move or position your eyes expresses something.