Here is a man with all the money in the world, a good looking wife, two or three nice homes.
WTF does he want to be President for?
For the power and GLORY of it.
Recall that his father, George Romney, ran for president, and lost. I think it's a case of the son trying to realize (or live up to) the father's dreams.
I'm sure that Mitt made this one his goals in life, a long long time ago.