When you're running for president "your own best interest" is to go down in history as one of the greatest and most popular presidents in history. If you're the president you're going to be rich, you're going to be famous, so trying to increase your popularity among the people by solving their problems as best you can is what's in your own best interest.
Now there are those particularly on the republican side of the isle who think that lining the pockets of millionaires and billionaires does some how solve the problems of the masses. They believe that warmongering is in the best interest of protecting the country. But this idea that someone runs for president just to line their own pockets is actually kind of stupid. There are far easier ways to get rich than that.
A typical government employee..I might worry about. A lower level congressman or senator...I can buy that, but once you reach the level of President, Vice President, Secretary of State, Speaker of the House, Senate Majority leader..... I mean I fucking hate Paul Ryan and Mitch McConnell, but I have no doubt that they honestly believe that they are doing what is in the best interest of the country as a whole even though they are completely 100% wrong.