Why is Trump refusing to publish his tax returns?

Donald Trump hates the idea of havng his tax returns made public and he and his accountants continue to stall over Democrat demands that he comes clean. The usual excuse is that Trump's finances are still being audited. I don't know whether American voters care one way or the other about his taxes but there must be some who are curious about whether he has paid any taxes at all which, being a billionaire, could mightily annoy those people who work hard all their lives and pay their taxes like dutiful citizens. Surely he must have paid some taxes over the years, even if his accountants have found dodgy but lawful ways of paying the minimum. As president he should demonstrate that he has met his tax oblgations like any other decent American. But there has got to be something murky in the Trump tax history. Otherwise he would have published them a long time ago. I thought US presidents were supposed to reveal all about everything. If they sneeze, the nation is supposed to hear about it. This is what happens with the Queen of England. The slightest hint of a cold or cough and we Brits get to know about it through Buckingham Palace spokespersons. And we know all about her taxes and investments and annual cost to taxpayers. But Trump wants to keep his business stuff separate from his presidential stuff. I say again, come clear Mr President because if you don't, the Democrats are going to make a song and dance about it all the way to the presidential election in 2020. Many of the Democratic presidential hopefuls have already published their taxes. Senator Kamala Harris is the latest to do so. She and her husband paid over $2.2 million in taxes in the last five years. Beng transparent seems to me to be better than looking and sounding shifty which is the case with Trump and his financial dealings. Voters should be told what their president has been up to in the past. But will Trump come clean? I doubt it.