Apparently to truly know if someone is acceptable for the very important job of Vice President it requires examining their tax returns for the past decade. From the Daily Beast:
As part of its vetting, the Romney campaign required at least some of the candidates on the short list—including the eventual winner of the GOP veepstakes, Ryan—to submit fully 10 years of tax returns, according to a knowledgeable source.
The requirement was consistent with the past practices of both Republican and Democratic campaigns. Indeed, in 2008, Mitt Romney turned over 23 years of taxes to John Mccain’s campaign when he was under consideration to be the Arizona senator’s running mate.
Romney felt he needed ten years of tax returns to pick a running mate, but he maintains that it is absurd for the American people to ask for more than two years of incomplete tax returns from a candidate for President.
By asking for ten years of tax returns the Romney campaign effectively admitted that what is in past tax returns could be relevant for selecting a candidate for higher office, even though Romney himself is still publicly saying two years is all the information anyone should need about him. If he honestly believes what is in past tax returns does matter, he wouldn’t have asked for them from his VP candidates.