And it's usually bad news for the villains of Gotham.
"I've never seen you laugh before... I don't think I like it."
This episode was actually really dark. Harley is clearly so dedicated to the Joker and he cares so little about her. He even throws her out a window!
Batman just has a weird sense of humor. One that involves very little laughter and a lot of unplanned skydiving.
So he probably makes US laugh more than he laughs himself.
And he's pretty much sworn not to laugh at anything the Joker does on principle if nothing else. It's probably hard being Batman. I'd be pretty humorless too if I was responsible for keeping all of Gotham safe!