Why do people have to be jerks?

I realize this question has many answers, and I would love to hear some of yours.

I guess I'm just naive, or stupid, or this heat is making me particularly sensitive (and cranky), but I am constantly amazed, and hurt, by how incredibly insensitive or thoughtless or mean people can be -- especially so-called friends. (I am not even going to touch on bosses or people we work with. That is a whole other blog post.)

Do people get off on being jerks or making others feel bad? Does it make them feel better making others feel worse? (Btw, those are rhetorical questions.)

Why when a friend tells you she never went to a prom and has prom envy would you say "I went to six proms! Proms are great!"

Why when a friend tells you she is on a diet would you eat a big hunk of chocolate cake in front of her and announce, "I love chocolate cake! Don't you?"

Why when a friend tells you he is really struggling financially would you say "Hey, want to go for a ride in my new Ferrari? I just got her, and she only cost a quarter mill!"

Why when a friend tells you he was just laid off would you brag about your promotion?

Why when you opine to a friend, "I'd love to be able to go out more," does he then go and on about all of the great restaurants and parties he goes to?

Why do people leave nasty anonymous comments on blogs?

And what is with all of the one-upping?

Let me know via the Comments.