I remember one of his campaign videos so vividly. He said, "I didn't want to have to run for president. I had a beautiful life; I built a beautiful company. But when I saw what was happening to our country, I had to do it. I had to do it."
I am generally a bit cynical about politicians, which perhaps comes from meeting too many of them. However, I have also known a lot of very wealthy people. I know that Trump didn't become president to get richer, and also not to get access to interns like Hillary's husband since billionaires have that option already. When he said that he "had to do it", I believed it was patriotism. The mystery was what he wanted to accomplish, and I imagine a lot of people are surprised to find that his campaign promises were all true.