https://www.cnn.com/2019/04/03/politics/donald-trump-lies-reasons/index.html

"Why does he do it? Because facts have always been a fungible thing to Trump. He has, throughout his life, bent them to fit the narrative he is telling about himself. For most people, getting something factually wrong -- especially in a public setting -- is disconcerting and embarrassing. Trump doesn't have that gene. Facts are whatever he wants them to be. And they can change, depending on his own circumstances. "