Stress
Don't you know it. He's got the whole world in his hands.
Have you ever seen a really great President's national approval rating? Only three presidents in the last seventy years have hit the eighty percent mark for even a brief time during their service. Most hover around fifty or sixty percent for the bulk of their term(s).
People get pretty passionate and heated about politics, meaning that maybe twenty to thirty percent of the nation out-and-out hates you. Another twenty percent feel "hate" is a strong word, but they think you're doing a crappy job. Roughly another fifteen percent approves of you only because they voted for you and don't want to be proven wrong.
As you might imagine, it's incredibly stressful dealing with that kind of pressure to perform monumental tasks that will turn a whole country around, and to be reviled or at least disliked by so many millions of U.S. citizens. Some people can't even handle it if their cube-mate at the insurance office gives them the evil eye once in a while.