Part 1
False certainty is a fool's certainty: it is based in the illusion that the more strongly we feel about any given moment -- in particular, the more negative we are over whatever may have transpired -- proves not only the truth of our unique perception... but also why we're so certain that whatever conclusion we reach goes well beyond the measure of any doubt.
Part 2
The reason that it never occurs to us to doubt our negative states is because to do so would require us to doubt our very identity in that same moment.