Goosebumps happen when a tiny muscle attached to each hair follicle tightens and pulls the hair upright, puckering the skin around it. In furry animals that reflex traps insulating air and puffs the coat up to look bigger — humans kept the reflex but lost most of the fur that made it useful.
For a furry mammal, this reflex does two jobs at once. Erected hairs hold a layer of still air against the skin, and that trapped air slows heat loss — the same reason a puffed-up jacket feels warmer. And when the animal is threatened, a fully raised coat makes its outline bigger, which can be enough to make a predator think twice.
Humans run the identical hardware. The muscles are there, the follicles are there, and the signal still fires when you are cold or startled. What changed over evolution is the covering: with so little body hair, standing it on end traps almost no air and adds no size.
The reflex is wired into the same fight-or-flight system that responds to threat, which is why a frightening moment can raise goosebumps even in a warm room. Many people also report them during powerful music or moving scenes — researchers are still studying exactly why emotional peaks trip the same switch, so that part of the story is not fully settled.
See the 20-second version: "Why Do You Get Goosebumps? — Things You Didn't Know" on LOE PRESSURE (TikTok @loepressure and YouTube @LoePressure).
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Sources: Encyclopaedia Britannica (goosebumps / piloerection and the arrector pili muscles); Harvard Medical School / NIH coverage of arrector pili research.