He isn't hiding anything. But I can tell you that I while my tax returns are squeaky clean, I still don't want to put out my personal information. Private is private. Stealing someone's private information is illegal and just because someone doesn't want to provide that information doesn't mean they are hiding anything.
But will tell you this, the Left is thirsty for something, anything they can spin to discredit or delegitimize his presidency. I have nothing in my taxes for anyone to challenge, but I would not put my tax returns out there for the vultures on the Left. They would lie, and misrepresent Trump's returns and he knows it. The Left, especially in the media are not honest, they are liars and would completely distort anything they could get their hands on. I would not trust anything to people who lack the basic quality of integrity that we have seen out of the Democrats on Capitol Hill and on the liberal media.
So, I don't blame him for deciding not to put them out there. He has been demonized, lied about, his family has been harassed, his supporters are harassed and assaulted on regular basis and members of his cabinet have been harassed in public after hours to the point they cannot even go out to eat.
So yeah, they pretty much disqualified themselves from any privilege of seeing his returns. They cannot be trusted to be fair or honest about what those returns would have in them.
He is not legally required to disclose them and no one has right to them.