Why so negative?

I really can't understand how some people can be so malicious and unfriendly. I just don't get it. I mean sometimes I suppose if you just had some sort of tragedy, but I mean gosh. That mean? In my writing class we just got done workshopping our pieces and there is this one girl in the class who can't go one story without saying something negative. Even if it's right down to something as small as she thinks the main character's hair color should be different. Yeah. I know. Today in class, she suggested that this kid change the entire concept of his story (an invisible man) because it did not seem realistic.

...

hello? This is a creative writing: FICTION class. Seriously, what the hell is up with people? Am I missing something? Am I missing the gene in my body that allows me to be mean to everyone? Or maybe being nice is not a human quality. Maybe it's only found in rare cases... like a disease. A disease that I wish everyone had. Don't get me wrong, I know a ton of nice people. There are many in my life. I am just questioning the attitude of certain people.