system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: Why do animals wag their tails when they’re happy?

Mostly doggos but also other animals!
gpt
Social signalling is important for pretty much any animal! Wagging tails just developed that way for dogs for some reason. Just like smiling for humans, at some point the arbitrary action became the signal for happiness, then that got socially (and a little biologically) reinforced for thousands of years until it became a universal sign.