what causes goosebumps?

Answer 1

They are called piloerections. Back in the day when humans were hairier, our hairs would stand on end when it got cold or when we got scared. Hair standing on end in the cold provided a better barrier against wind and cold and when people were scared, it made them look bigger to a potential threat.

These days, we have evolved and are not quite as hairy, but the human response to such stimuli is to create the "bumps" that caused the piloerections (hair standing on end).

Answer 2

fear, change in temperature, allergies...