I want to see what Trump earns and how his accountants and lawyers move his money around. I want to see what he donates. Why is he is hiding his taxes. That makes me suspicious.
Trump has a low opinion of Romney. Four years ago Romney was pressured into revealing his tax returns and I think that's one of the reasons Trump sees him as a loser. He buckled. Another situation where a politicians is dammed if you do and dammed if you don't.

Poll: Public supports disclosure of tax returns for presidential candidates - Sunlight Foundation Blog

68% of the people polled agree with me.