We were playing at Bally's. Short version of the story: I had suited Q-10 against an aggressive player's K-J. We both liked the K-J-9 flop, but I liked it a little better than he did. I bled him for about $105 before he cried uncle to my big check-raise on the turn. He showed his top two pair before folding, and in return I showed him my straight.
My default is not to show when it's not required, but that's not an unbreakable rule. See section 4 of this post for a brief rundown of various strategic ways of approaching the show/don't show question.
In this particular instance, there were two forces at work:
1. Social/informational. A couple of years ago I discussed the voluntary exchange of information that sometimes occurs at the conclusion of a hand that has not gone to showdown. This was such an occasion. If I don't at least occasionally reciprocate when another player initiates this kind of exchange, they will stop doing it, and I'll lose out on the information I gain from them voluntarily showing. I thought this guy was one of the most unpredictable players at the table, and I was happy to have a chance to mentally correlate how he had looked and acted during the hand with what he was holding.
2. Metagame. At that point in the proceedings, I was feeling that I was being called down a little more than I wanted to be, not being given enough credit for having a hand. This was a rational reaction for the table to take, given that I had been playing some marginal hands and had been pushing the trickiness a little more than usual. Showing the nuts was intended to regain some respect, so that I could resume using valuable tools such as the check-raise bluff and the light pre-flop three-bet. And I'm pleased to say that it worked. In the next half-hour or so, I picked up a few pots with aggression not supported by the cards I held.
So that's the answer. Not particularly brilliant, original, or exciting, but the truth.