I have discussed before the surety that psychotic people feel in their delusions. I may not have mentioned how intense it can be, that they cannot allow you to get even two words into a sentence where you are disagreeing - as if they cannot even endure that an opposing view exists. It is a little puzzling. People who disagree even violently about political or religious points can usually make themselves at least listen until another person completes a sentence. Guys getting into bar fights about nothing usually at least get complete idiotic sentences out, which are responded to with at least some minimal comprehension. People with OCD can at least hear you ask "What is the bad thing that will happen if you don't count the squares again?"
My only theory is that because the place in their brain which makes and compares alternative narratives is so thoroughly broken that any bumping up against it is psychically painful, like bright light in the eyes of a person coming from complete darkness, it is perceived as a denial of reality at impossibly levels. It is not a questioning of whether the sky is blue, but of whether there has ever been a sky.