Your question gave my mind all sorts of virtually instantaneous humorous thoughts.

1.) Perhaps disembodied asses are known to break wind uncontrollably, and he didn’t want to have to give any embarrassing explanations to future dinner guests?

2.) Maybe it’s animated, like “Thing” on the Addams Family, and is given to insistently rattling around in an annoying manner when it craves attention?

3.) It can get mighty cold when you don’t have any pants on?

4.) In order to protect it from getting its feelings hurt, he doesn’t want it to hear him having sex with other young ladies when he brings home dates, and gets lucky?

5.) The guy who owns it was going at it one day hot and heavy, and suddenly discovered (in the worst possible way), that while it was stored away, it had somehow picked up a very sharp splinter from the previously inner wooden surface of the container, and he certainly didn’t want THAT to ever happen again.