Why do we laugh when someone tickles us?

Answer: It's a largely automatic reflex that probably began as play

Tickled nerves short-circuit into the laughing muscles — Nothing short-circuits: tickle signals travel on ordinary touch and pain nerves to the brain. The laugh comes from brain areas that also handle play and surprise, which is why the same touch from your own hand does nothing.

It's a largely automatic reflex that probably began as play ✓ — Tickle laughter is largely automatic; people laugh even while trying to escape, and you can't tickle yourself because your brain predicts your own touch. Many scientists think it began as a play signal, since young apes laugh the same way during rough-and-tumble games.

Tickling releases a chemical that makes you happy — Tickling doesn't reliably make people happy; many laugh while squirming to get away. That mismatch shows the laugh is a reflex-like reaction rather than a sign of pleasure.