People usually don't get upset when something said to them is simply not true

If somebody says something to you, about you, in a way that is meant to put you down, but what they are saying simply is not true, the natural reaction to that is not to get upset, but to brush it off and not even worry about it. People only get upset when you call them out on something that is true.

The President of the United States flipped off and yelled expletives at an individual citizen when that citizen said something to him. Think about it, if what was said was not true, there wouldn't be any reason for the President of the United States, of all people, to get that upset about it. I think Trump was caught off guard and it ruined the moment for him, which is what likely irritated him to the point of lashing out in response.