Why do we say things we don't mean?

Then i am a total fucking idiot.

Why do you say shit if you don't mean it? Like, unless you were trying to trick someone, or prevent yourself from getting in trouble, which I understand, why else would you intentionally say something that you don't mean?

@BlueMetalChick:i don't know. If i am angry at someone i say something i know will hurt them and then i regret it. I didn't mean to say it.