I believe it's because otherwise, he suffers a narcissistic injury, and he must defend himself at all costs against losing his false sense of self and false inflated ego. If I point out everything wrong with him and confront him square on with who he truly is, his ego gets severely punctured and he will fight against that. So he must try to prove to me otherwise that he's such a "good person" and "good guy". With a narcissistic injury, they lose that inflated false sense of self and can plummet into severe depression, self destruction and loss.