^ Mostly because they put their self interest ahead of other people's. Most people don't enjoy it (there are sadists, but they are much rarer). Very often, people don't realize that they hurt other people. Actually, if we think about it, all of us may have already done it at least once in our lives. Also, hurting is a very subjective concept, some people sometimes feel "hurt" or offended for nothing...
All in all, it's complicated question... here is what I should have answered