Not to mention that being the President is a job that nothing and no one can nor is really prepared for. For all the posturing made during the most recent election on having the experience to be the President, the only thing that can really prepare one for something of that magnitude... is being the President. Not even being VP is proper preparation... if it was than ever VP turned President would have been elected for a second term, but we know that's not true.
Basically you've got the lives of hundreds of millions of people, if not more, on your shoulders every day. You make decisions as well you can with, at best, broad advice from previous presidents (because none of them ecountered situations exactly like they one you're facing). And what's worse is that failures by those under you get blamed on you too, and that all of the mistakes that you make or that trickle up to you from the failures below are all visible on the entire world stage and pretty much for the rest of history.