OK, it’s not just you. Most people are really mean to themselves. If you take the time to examine how you speak to yourself, you might notice this.
In your head, do you talk to yourself like you do your closest friends? Or your dog?
Probably not. I’m guessing that you generally give your friends the benefit of the doubt. Most likely you are kind, tactful, and supportive. When they are feeling down you try to build them up, and you try to help them see themselves more compassionately.