It comes with the extreme narcissism IMO. If it comes out of Trump's mouth, it is therefore true, and if he contradicts himself 15 minutes later, that is also true.

Trump used to have some semblence of filter (even referring to his exaggerations as "truthful hyperbole"), but as he's getting older, those filters seem to be diminishing in effectiveness.

There is a massive difference between a contempt for TELLING the truth and a contempt for the actual, empirical existence we all face.

Trump is the latter.  His lies are not to gather support for a policy as many POTUSes have done ("if you like your doctor, you can keep your doctor",WMD in Iraq, etc.).  His lies are not to protect his own skin ("I did not have sex with that woman, Ms. Lewinsky").  His lies are completely and utterly to stroke his own ego.  Any fact that demonstrates that he is less than he perceives himself is simply dismissed no matter how absurd the dismissal, no matter what person . . . of power or the down-trodden . . . he has to run over to put his ego-stroking lie forth.

This is a man with a pathology.  A man with his finger on the nuclear trigger.