Both Clinton and Kaine have released their tax returns. Pence says he'll release his tax returns. Trump should follow suit and release his own returns. At this point, even if he doesn't have anything to hide, it's starting to look like he does, or at least look like he's not comfortable with it. He's said quite enough loopy cringe-worthy things, that at this point, it's not gonna make it much worse.
Indeed, political candidates generally release their tax returns in order to convey a sense of honesty and transparency. Donald Trump's reluctance to do so indicates to many that he has information he wishes to keep from the public eye. As his numbers in the polls have been dwindling, doing an action such as this may help his campaign.
Donald Trump has characterized himself as an incredibly successful and charismatic businessman. The charismatic part has proved to many to be false, given that he has the complexion of an Oompa-Loompa and the hair of a mangy squirrel. A release of Trump's tax returns may prove the 'successful' part false as well.
Donald Trump emphasizes he will not be able to discharge his income tax earnings due to the fact that he is being subjected to an inspection that will go back 4 or 5 years. Unfortunately, polls are demonstrating that Donald Trump could possibly have a difficult situation to deal with in the event that he declines to issue his income tax earnings.