Donald Trump is incapable of trusting anyone. He has contempt for the people who serve him faithfully. He seems to think he is still the head of a privately held company, but he is not, he is the chief executive who serves at the pleasure of the voters.
It reminds me of something I read a while back - that Trump believes the number one trait of any supporter is loyalty. Not loyalty to America. Not loyalty to The President. But loyalty to him, Trump, personally.
If you think about his actions through this filter - just about everything he does, to attack other people (which is incredibly undignified of a President, but that's a different matter) starts to make sense. All the people he's fired, and what he's said about them, starts making sense. His expectations of others, that they lie for him, that they attack other people for him, that they stand by his public statements no matter how out there they are...starts to make sense.
More recently, I've started reading 'The Dictators Handbook'. A rather interesting read so far. One of it's central concepts is that rulers that last a while (whether Presidents, Dictators, Kings, CEO's etc) want / need / seek people that are loyal to them personally. The more autocratic they are, the more they need personal loyalty to stay ruling. Of course it's much more complex than that, but it is interesting just how much Trump acts like their examples of dictators, rather than their examples of Presidents.