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