Why do we get goosebumps?

Goosebumps happen when tiny muscles called arrector pili pull your body hairs upright. Cold, fear, or strong feelings can trigger that signal through the nervous system, puckering the skin around each hair. On people, the hairs are too fine to make a warm coat, but the reflex still shows.

The old purpose made better sense with fur. A colder animal could hold warm air near its skin, and a frightened one could look larger. Cats still puff; birds lift feathers; a porcupine can raise a spiky warning system that we mostly feel as bumps.