Why on earth would a candidate not want to show their taxes? Donald Trump said himself that there has to be something wrong if a presidential candidate does not want to show his taxes. I guess he did not realize he would be running for the highest office a few years later. For some reason the game has now changed in his head and he does not think he should show anyone anything. He plays by his own rules apparently.
The practice of presidential candidates releasing their tax returns publicly has been a practice since the Watergate scandal and one that helped promoted transparency in the presidency. It is an important ritual in our political process because it shows that the candidates are willing to be open to public scrutiny on this level, namely, the ways they make their personal income and how much that income is.
Releasing tax forms should not be a requirement of presidential candidates. Tax forms contain private, conditional information and they should not have to release this information to the public. However, if a candidate bases their campaign about this type about this information they should release the information and answer to the voters.
No one should be required to publicly release their tax return. That seems like a breach of a person's privacy. However, if a presidential candidate is asked to release it and refuses to do so, it looks like the candidate might have something to hide. This may seem slightly unfair, but if you want to have one of the most important jobs in the world, transparency never hurts.