Why do the hairs on your skin stand on end when you feel excited?

When you feel excited, your body activates the sympathetic nervous system, triggering the "fight or flight" response. This causes the release of adrenaline, which can lead to the contraction of tiny muscles at the base of hair follicles, known as arrector pili muscles. As these muscles contract, the hairs on your skin stand on end, a phenomenon known as piloerection. This reaction is a vestigial response that may have helped our ancestors appear larger to predators or conserve heat.