Part of it is narcissism. I see the word thrown a lot, to mean a lot of things. A major component is unable to face being wrong. If it comes out his mouth, he believes it to be true, right down to his roots.

I was 40 or so until I realized my mother had never once in life said she was sorry or admitted fault. One time my ex and I nailed her to the wall with her own words. You could see the cognitive dissonance boiling away. And compared to her, Trump has weapons grade NPD.