What is Romney Hiding?

So Mitt Romney wants to be President but he doesn't want to show people his tax returns. Is it because he has something to hide? Is he afraid if people see his tax returns they won't vote for him? All the more reason for people to not stop asking to see his returns. This is from Edward D. Kleinbard and Peter C. Canellos at CNN:
The U.S. presidency is a position of immense magnitude and requires a thorough vetting. What the American people deserve is a complete and honest presentation by Romney of how his wealth was accumulated, where it is now invested, what purpose is served by all the various offshore vehicles in which he has an interest and what his financial relationship with Bain Capital has been since his retirement from the company. These are all factors that go to the heart of his character and values.
It's quite probable that Romney has hundreds of millions of dollars stashed overseas. Why would he do that?  It's possible he's a tax cheat. Why else would he hide his returns? A lot of people are not voting for Romney because they like him, they just hate Obama.  So yeah, some people hate Obama so much they are willing to vote for a possible tax cheat. Romney should know that in the absence of information, people will tend to make up information to fit the narrative.

So people will continue to ask for Romney's returns.  After all, many people never stopped asking for President Obama's birth certificate, right?  The drumbeat will continue.