Donald Trump is not releasing his tax returns because he fears it would show he is not as wealthy as he claims. He is someone that has bragged about his personal wealth, and he has exaggerated it by including billions for his personal brand. Others have also made the claim that his net worth does not live up to his claims.
By now, we should all be well aware of how narcissistic, yet fragile, Donald Trump is. He hates nothing more than to be attacked and made fun of, and for people to make fun of him for not being as wealthy as he claims would hurt him deeply. He definitely wants to avoid that.
It's possible that his returns would show he's not as wealthy as he claims. It's also possible that they'd show he's making money from some Russian/Ukranian business connections, or from some hidden business that could be traced to one or two of those types of sources. Either way, not releasing his tax returns makes it look like Trump has something to hide.
Trump not releasing his returns would not show that he is less wealthy than he claims. Then again, who cares? It is no one's business what is in any other person's tax returns. This is nothing but political posturing of the worst kind. The best part is, the more people attack Trump, the better he does.