In an interview on Meet the Press, Trump asserts that Mitt Romney lost because he released his tax returns. Therefore, this is why Trump will not release his returns.
While he is correct that some people used Mitt Romney's income against him, this is rather lame excuse for Trump's hard line against providing his tax returns. The position that people voted against Mitt Romney because he has been too successful in business and finance flies against all logic and sensibility. Looking at exit polling data and other information from 2012 points to many other issues, most of which include comments the were portrayed as Romney saying, "I think 47% of American's are free-loaders". As a free-loader, I think that was actually a correct observation, but alot of people found that offensive.
Trump's assertion that the tax return was too damning for Romney's campaign is a red herring. There must be other issues in play.
Early on in the primaries, Trump mentioned that his personal tax return is under audit and has been under audit every year. Contrary to Trump's assertions, the IRS doesn't have a policy of auditing people on an annual basis simply because they are rich. They audit people annually because they see smoke (you know the old saying, "where there is smoke, there is fire") in the individuals financial behavior and record keeping.
This website (despite possible political leanings) makes some good observations about possible reasons why the Donald cannot get off the IRS radar. Long story short, the IRS audits Donald each year because his business and personal conduct have major badges (red flags) of concealment, under-reported income, and over-inflated deductions. The IRS is run in a business like manner, IRS audits against Mr. Trump would cease if he managed to go through an audit without having major adjustments in the governments favor. However, as IRS revenue agents find concealed income and illegitimate deductions on an annual basis, the IRS (like any business with a new revenue stream) opens up each successive year.
Here is another article by a corporate tax lawyer that theorizes what we might find on Trump'a return.
I admit that all of this is speculation from (at least) a pair of tax professionals, and maybe there is some weight to a "Obama orders the IRS to audit Trump" conspiracy that tin-foil hat wearing Trump supporters would love to believe. However, the fact is that unless he releases his returns, the theories are going to continue with greater intensity. What will the orange man do?