They wonder whether their turtle is lonely. Or they would like to get a second turtle for breeding. Usually, turtles that people acquire are babies or juveniles, and they get along well. However, as turtles mature, and their hormones kick in, more often than not they will start to bicker and pester each other, and then you are stuck with one setup and a number of turtles that don't get along. Your turtle does NOT need a companion to be happy.
Turtles do enjoy company of other turtles, and, of course, turtles enjoy the mating game.
However, unless you are willing to provide a large enough enclosure for the turtles to hide from each other, or separate the turtles during breeding season, or maybe even for most of the time, don't get another turtle. Trust me, I've been there and done that. It seems clear that for some reason the new environment let the male realize that he was, indeed male. Unfortunately, sometimes we want to do good by our turtles and provide them with a new and improved environment, and it backfires. Yes, I can understand that you are sad that this didn't work out.