Jonathan Bernstein speculates:
[O]f the modern presidents, the one that was least ambitious was probably George W. Bush, and that's a good part of why he was a terrible president. It's rare to reach the presidency without aiming at it one's entire life.
Bill Clinton, as far as anyone can tell, was aiming for the White House from at least high school on. George H.W. Bush was ambitious for a long time. Ronald Reagan, remember, ran for president for about fifteen straight years before finally achieving it. And one could argue (indeed, I would probably argue, although as I said it's pretty speculative) that lack of intense ambition was a real problem for the younger Bush in the White House.
Would a more ambitious president, one who was really desperate for the job, have found himself fighting two wars in a haphazard way in an election year? Would a more ambitious president have been so apparently indifferent to the fate of New Orleans? Now, ambition isn't foolproof, as a quick look at Richard M. Nixon will show. But the greatest presidents were certainly quite ambitious. In my opinion, you can't be a great president without it.