It often comes from one of two kinds of people:
People who give themselves feedback in the same heartless tone. They don’t hesitate to brutally lash out, because that’s the noise they often hear inside.
And folks who honestly believe that their work is flawless. They can’t understand how anyone can else can fail to measure up, because they never seem to.
Of course, each group has a significant (though different) problem. In fact, now that they’re spreading their harshness with others, they have two problems.
When in doubt, look for the fear.