Batman inspired the citizens of Gotham.
One of the inspired ones was a man deciding to rise above like his hero, but to prove his hero wrong in one key aspect: Batman seemed to believe the normal and weak, civilized people to be good.
Joker became a deranged fan of the man bat. He finally saw a fellow thinker, greater than the herd of weaklings. He admires Batman -his best friend. Batman is the best thing Gotham produced, an incorruptible force. Joker is the worst thing Gotham produced, a lawyer and hero for evil, seeking to corrupt.
Joker despises the criminals more than Batman, he sees them as puppets without a human value, just like he sees everyone else. To him the law abiding citizens are no better, they’re just a crime waiting to happen.
In his world Batman and Harvey Dent have the most value, they’re the only ones -with him- to have real agency, only ones above the herd. Joker doesn’t believe in the existence of Good, he doesn’t see any value in life so he’s ready to die. He wants his best friends to understand why, he wants them to join him, he wants to socialize with the greatest men he has seen.
If he would have to watch the world burn alone, the spectacle would feel empty. He would be a philosopher without an audience. Only Batman is on a high enough level to have a conversation with him. Joker feels curiosity towards Batman, and if he would be capable of love, it would be towards Batman: an incorruptible force standing above the herd, above the normal. A man who dresses up to look like a demon but who for some incomprehensible reason fights for the people who don’t deserve him.
Joker is happy when interrupted by the Bat. He counts on it, he needs his friend. In his chaotic world a difficult friend like Batman is a gift. He has no ego when it comes to fistfights with the demonic bat, he admits without any pain his friend to be superior in strength and skill. Out of everything he has seen, Batman is the most interesting thing to study and to interact with.