The moment that Batman stopped reacting was the moment that the Joker's laughter ceased.
Most of the fun in being outrageously evil came from watching the caped crusader get all riled up, which is why the Joker had his best jokes lined up, the most despicable things he could think to say, the most haunting threats on the lives of those Batman dared to care about. But one day, those things stopped commanding the gritted teeth and balled-up fists.
Joker upped the ante. Bigger crimes, more heinous comments, louder laughter. A henchman or two died in the process - newbies who nobody warned against attacking Batman without explicit instruction ("That's what they get for ruining my punchlines. Anyone ever heard of comedic timing, for Pete's sake?").
"It's like paying to take a girl to dinner and getting nothing out of it!" he monologues to Batman once in the midst of a confrontation. "But, you wouldn't know what that's like, would you, Batsy?"
Batman seems more upset than he's been in a while. That's not saying much, but it's a start.
The analogy doesn't reflect well on what the Joker thinks of dates, but it's not news to Batman. Despite that, he can't help but wonder if the mad clown meant that he never goes on dates, or that he never gets rejected. He's not sure why he cares, so he tells himself that he doesn't. He's just tired (but it's a poor excuse when he knows he's never not tired).
The flinch is enough to have Mr. J grinning maniacally. A chink in the armor was all he needed, after all.
"I set up all this-" he gesticulated wildly, referring to the burning building behind them, "-and you can't even pretend to care! Oh, why do I bother?" The Joker pressed the back of his hand to his forehead as if he might faint. He's made of melodrama.
"You must care about some other villain more than me, don't you?" he pouts, or the closest thing his face could ever get to it. "Is it that damn cat? Do I need to teach someone a lesson?"
"Shut up," Batman hisses. It's more than enough to send the Joker into a maddened fit of laughter, a high he takes several seconds to come down from.
"Oh, so it isn't true! I...heh heh heh...I should've known."
The smile on his face is dripping venom. Dangerously lanky for someone his size, he seems like ill-intent and nightmares shambling around in a vague approximation of a man. He steps closer to the Bat, who looks more than ready to raise a fist.
"You only have eyes for me, don't you?" He brushes rubble that Batman can't see off of his purple jacket, strides as steady as ever. Another laugh starts up in his chest, though it's really more of a rumbling growl.
"It's always me, isn't it?" He's almost threatening Batman to disagree with his menacing tone. "Always have to leave room in your heart for yours truly, am I right?"
In a second, Batman's hands are around the Joker's neck, taking the bait easily. It's an intense display of self-control when Batman doesn't squeeze hard enough to kill. Then again, killing the Joker is never in the plan, is it?
"Kinky," Joker wheezes out. Batman drops him like he's made of fire, disgusted at the insinuation.
The Joker laughs. He laughs until he cries, and, if he didn't know who he was dealing with, Batman would wonder how anyone could find this much amusement in these moments.
He laughs as Batman restrains him. He laughs on the way back to Arkham. He laughs when they stuff him back into his cell.
If he wasn't the Joker, they would've asked him why. And oh, how he would've told them about the wonderful evening he spent with Batsy.
No one asked, of course. No one wanted to know.
A/N: I usually find it easy to imagine the Joker flirting with Batman just to piss him off as much as possible. This is simply a manifestation of that idea.