The Joker is the ultimate arch nemesis of Batman. Batman sees the ills of society and the need to bring reform outside of social norms; but, he still sees the need and good in social norms and society as a whole. The Joker sees presence of random injustice means there is no justice. The fact that innocence can be destroyed means there is no innocence. Therefore your life, pretending these things exist, are a joke. When someone says your life is a joke, there is a challenge, a physical challenge, a moral challenge, an intellectual challenge. Therefore the Joker isn’t just threatening Batman physically, he’s threatening the premise of Batman’s existence. That’s why it’s such an epic discussion they are having through their physical confrontations, mental games, punches, and gunshots. Ultimately it’s a philosophical conflict, and one that’s not easy to resolve.