Trump has been asked to share his tax returns numerous times. Yet, he still hasn't done it. But why? While all of the following could be wrong, if he's not sharing them, there must be a reason for it. And the true reason can't be the one he's stated himself - that he can't release the tax information because it's under audit. In 1973 Nixon's taxes were under audit as well, he released them anyway.

He's not as rich as he claims to be

As for Trump it's all about the looks, all about the impression, he wants people to think he's a self-made (which he's not anyhow) billionaire. He wants people to believe that even if he's actually not a billionaire, and maybe actually hundreds of millions in debt. Releasing his tax returns would burst the bubble for people who are believing he's worth 10 billion dollars. Maybe he ran for president just for one reason - to promote his brand, because he was failing miserably in business and he thought becoming the POTUS would be the only way to turn it around.

He doesn't really pay taxes

Trump has always bragged about being the smartest guy on earth, and that he's able to game the system. So maybe he's actually not paying taxes almost at all. And that would again hurt his self-view, considering his self-view requires enough people to look up to him (although, nobody should).

Crooked sources of money

Maybe his money isn't coming from as legit sources as one might expect. Maybe a lot of it is related to Russian oligarchs, maybe mobsters (doesn't matter if US or Russia based), maybe from his charities people send money to for theoretically a good cause.

Whether one of these versions is correct or not is anybody's guess, but we all know that he lies close to every time he opens his mouth, while claiming he's not lying and saying these are facts (that...well, he just made up), he cheats (I hope you haven't forgotten about Stormy yet) while claiming he's not cheating. He's crafting the myth about the Legend of The Donald, and everything he doesn't like, he claims is wrong, fake news.