https://m.washingtonpost.com/blogs/ezra-klein/wp/2012/11/04/why-does-mitt-romney-want-to-be-president/
This article gives a really interesting, and for the most part, non-partisan perspective on why Romney might want to be president. The question of why Romney doesn't seem to have have a core set of his own beliefs is explained in an interesting context. From the article:
The glorification and subsequent commodification of "management" and "leadership" skills isn't confined to the world of superstar CEOs. You can see it in the business section of any Barnes & Noble. Rows upon rows of books promise to help you "get to yes" or turn into a "one-minute manager" or develop "strengths-based leadership." None of these books particularly cares what it is you're managing, or where it is you're leading, or why exactly you're trying to get to yes. These are skills like running or weightlifting. You can excel at them no matter where you're going or what you carry.
What Romney values most is something most of us don't think much about: management. A lifetime of data has proven to him that he's extraordinarily, even uniquely, good at managing and leading organizations, projects and people. It's those skills, rather than specific policy ideas, that he sees as his unique contribution. That has been the case everywhere else he has worked, and he assumes it will be the case in the White House, too. When we look at Romney's career and see a coreless opportunist, we're just looking at the wrong data.
This is why Romney thinks he should be president. A lifetime of data has proved to him that his management skills constitute a unique and powerful contribution. In Romney's world, there's nothing strange about that, which may also explain his willingness to be unusually strategic, even cynical, about the policies he supports.