Long of the short of it, Trump knows he's a fraud.
Politico interviewed several biographers who have followed Trump for most of his garish, boorish life.
D'Antonio: Those early influences are essential, and I also think it's correct that he has been conducting his entire life as a vanity show, and he's been rewarded, most recently since his reality TV show, by ever-greater public interest in him. This is a guy who is a president-elect who describes himself as a ratings machine, which is an absolutely absurd thing for a president to be reflecting on, but that matters to him.
But one thing I think that we have overlooked as we see Trump trying to delegitimize others is what I suspect is a feeling he has inside that nothing he's ever achieved himself has ever been legitimate. This is a person who has never known whether anybody wants to be around him because he's a person they want to be around or they want to be around his money. And since he's promoted himself as this glamorous, incredibly wealthy person, that's the draw he's always given. So he doesn't know if he has any legitimate relationships outside of his family, and that's why he emphasizes family. … He's always kind of gaming the system—not, in my view, winning on the merits. And even his election was with almost 3 million fewer votes than his opponent. So he has this deep fear that he is himself not a legitimate president, and I think that's why he goes to such great lengths to delegitimize even the intelligence community, which is the president's key resource in security, and he's going to do this demeaning and delegitimizing behavior rather than accept what they have to tell him.