I think a bully's mind works differently from others. Maybe they can't comprehend how distressing it can be for the other person, or maybe they just enjoy the feeling of imposing themselves, simply because they can. Sometimes people try to make excuses for them, like their home life or whatever, but I don't believe that. Anyone who harms another person, physically or verbally, with no provocation, is just a bad person. No matter how angry or upset I am for whatever reason, I would never take it out on another person.
I was really sensitive when I was younger (still am I guess), so I'm lucky it was never really an issue for me. If you ever want to talk or something, I'm here for you and I'd imagine everyone else here is too.
"Anger is an acid that can do more harm to the vessel in which it is stored than to anything on which it is poured." - Mark Twain