>>104209602>Why do people constantly say this?Context clues.
Panel 1: Batman and Joker laughing. Batman's dialogue directly under Joker's.
Panel 2: Batman and Joker laughing harder. Still two lines of dialogue denoting laughter, implying the two laughing over one another, with Joker's laughter above and Batman's below. Batman's hands suspiciously close to Joker's throat.
Panel 3: One line of laughter. Specifically, the lower one, implying it to be Bruce's- and that he's laughing after the Joker's stopped. In a word, improbable, unless something forced Joker to stop- like a strangled windpipe.
Couple that with all the commentary about how they'll inevitably see one kill the other, and that they've long since gone past the point of no return on the matter.
