His political advisers were against the pick. Romney was for it, because
Paul Ryan is not Sarah Palin. He is much smarter (without being really as smart as his reputation suggests) and a much, much better
Palin exposed McCain's impulsive, reckless side - which was only reinforced by the bizarre suspension of his campaign in October. Ryan exposes Romney's inability to reach out beyond the narrow band of people that he is comfortable with. You know, rich white people.
UPDATE: Gee, I wonder what his advisers were worried about?