Why are we so mean?

 Why is it that we are always the hardest on ourselves? My older sister and I were discussing depression and how we expect ourselves to be a certain way and that we often listen to the voices in our heads. I am very guilty of it! I find myself listening to those voices more and more. They aren't nices voices, they aren't kind voices, they are mean. I know that I would NEVER speak to someone else, the way I speak to myself. Why are we okay with it?
 I really like Pink's music and thought this song was very fitting. I have to remind myself that I am better then what the voices in my head are saying. It's rough, it's tough, but it must be done!