I think he thought people were patriotic and loved America enough to put their differences aside. He was wrong. I remember when the debate for replacing Scalia was going on, Trump went to meet GOP senators and Ben Sasse or his double from Arizona were there and acting rude. Trump apparently asked, do you really hate me that much? He was surprised they did not want to do what was best for the party and country.

Same thing with Mattis caring more about staying in NATO than replacing bad trade deals. Trump was surprised by it.