Why do people steal?

This is a serious question. I have never stolen anything in my life and do not even .01% understand the impulse behind it.

I am sitting in my campus pub right now. At the table next to me, a girl just walked away sobbing hysterically because somehow, someone had managed to steal her laptop. She cried, “My whole life is on there.”

Did the person who steal the laptop not know that? Know that and not care? Know that and decide his/her need was greater?

Can anyone explain this?