1.We want to insult ourselves first if we think someone else is about to do it.
2.We want to announce our deep awareness of our obvious flaws.
3.We never think we fully deserve praise without some silly disclaimer.
4.It has simply become a habit.Example:fat jokes,absent-minded/bad memory jokes,etc.
WHAT I HAVE ZERO UNDERSTANDING OF IS:
Why do people put OTHER PEOPLE down?
(Do you think there is a direct correlation between being put down by others and over-criticizing oneself?