It's a leftover mechanism in our evolution. Our ancestors had very visible fur, so when they were agitated or afraid of an approaching predator, their hair would stand in order to give the appearance they were larger than they really were. This increase in apparent size would normally scare off the predator. This feature stuck around, even though we don't really need it. The mechanism by which this occurs is pretty fascinating. Within each hair follicle, there are tiny muscles. Normally, they are relaxed, which is when our skin is smooth, and our hair is flat. But if we were to get nervous or afraid, the muscles contract to make the hair stand