For an ambitious politician, a spot on your party's presidential ticket is fraught with danger. On one hand, you immediately become a national figure, and if you win, you're vice president and you've got a good chance to become president. On the other hand, if you lose, you may wind up the target of contempt from forces within your own party and quickly fade away. Look at the list of recent VP losers: Sarah Palin, John Edwards, Joe Lieberman, Jack Kemp. None of them had any political future after their loss.
And then there's Paul Ryan. You have to give him credit for one thing. Unlike, say, Palin, he didn't let his time on the national stage give him delusions of grandeur. Instead of proclaiming himself the leader of a movement, he went right back to what he was doing before: using the budgeting process to push an extraordinarily radical agenda, all couched in enough numbers and figures to convince naive reporters that he's a Very Serious Fellow, despite the fact that his numbers and figures are about as serious as an episode of The Benny Hill Show. But this act is what got him where he is, and he seems to have concluded, probably wisely, that his best move is to get back on that same track, which might eventually lead him to the White House.