You represent hundreds of thousands or even millions of people—and every single stinking one of them wants you to do it their way. You'll need to make very powerful financial contributors happy, which may make other very powerful financial contributors upset. You have to be nice to people you really don't like.
There are only twenty-four hours in a day, but some days you'll feel like you'll need an extra dozen or so just to get some sleep.
Everything you do and have ever done is available for the entire world to see, hear, read, watch, judge, and mock to no end. All of your deeply-held beliefs will be challenged by people who can't believe you believe that stuff.
Even if you're the best spouse and parent in the world, your family life will probably suffer. You have to make speeches where one mistake or notion of inauthenticity could be the end of your entire career. You have to stand up for your beliefs every time you stand up.
Oh well. Making decisions that affect the rest of the nation should be stressful, and that's kind of what you're signing up for here.