I think beacause we have such good imaginations we think alot about what an ideal response or behavior would have/ should have been so when we are only pretty good we fall short of what could have/ should have been the best in our mind.
Perfectionism torture begins with a great imagination of what could be and somehow morphs into what we think "should be" because if we could think it we should be able to do it.......
and if its not the best we can think of, then its not good enough..