Why are People so Mean?

I just saw a post of a really pretty girl, asking if she were pretty, and almost everyone told her she was ugly, and trust me, she wasn't. Then her question was deleted, I asked the same question before just out of curiousity, sometimes people need more urance. Maybe this girl needed more, but, instead they called her a plastic barbie doll, not pretty, on and on.....What is up with these people? Why do they feel the need to be so mean?