There are other threads on the website about this similar topic, but just to give a quick reply:
During slavery, slaves were taught by their white masters that their dark skin and kinky, coily hair were ugly. Light skin was considered ideal and straight hair was considered "good" hair. Unfortunately, this belief has been carried down throughout the centuries. I've gotten curious looks and whispers from some other blacks when they see me in the street while most whites haven't paid me any mind. Blacks in general react more violently about hair because of the ingrained issues about it that have been passed down through time. It often isn't just about hair to us. Hair is often seen as a larger statement about our culture, which other people sometimes don't care to understand. Many black women relax their hair, but most white women aren't coveting locs or fros, which says a lot about how the two groups view themselves to me. Black women relax their hair to attempt to fit into a narrow standard of beauty and white women don't feel the need to imitate our hair, because they feel their hair is acceptable as is and/or is the "more acceptable" hair.