Why goosebumps?

An evolutionary guess as to why humans get them

People seem to get goosebumps when they are cold or frightened ("his hair stood up on end"). When a furry mammal gets cold, their fur gets fluffier (each hair in their coat is pulled upright, so it stands up more) so it traps more air and they stay warmer. Similarily, when a furry mammal is frightened (like a cat), they will puff up their coat so they look bigger an more intimidating. Perhaps humans have this evolutionary leftover, and so when we get cold or scared, we get goosebumps (ie. pull up on our body hair follicles). However, lacking much body hair, the effect is rather unspectacular.