The Lesser Evil wrote:The Joker is hard because his nature is so fluid, and he has the whole "multiple choice past" thing. And it's hard to pin a curse with him- of all of Batman's enemies he's one of them who seems to actually suffer the least- because it's all a game to him.
Unlike a lot of Batman villains, he's not visibly cursed with a mental vulnerability (like Twoface, Riddler, the Ventriloquist, Bane, Poison Ivy, etc. are), a physical weakness (like Clayface, Mr. Freeze), nor even unfulfilled social ambitions (the Werewolf, Penguin, Calendar Girl). He's Chaotic Evil to the point he's almost a figure of legend like a Bogeyman more than an understandable human mind.
The only way to make the Joker suffer, is to play up how "unfulfilled" life leaves him these days. He's been killing people for so long that it barely even gives him a rush any more. The only thing that makes him sit up and take notice is when Batman shows up to try and stop him.
Batman and the struggles Joker have with him are the Joker's only source of challenge, only source of joy in life.
The Joker: WHERE IS HE? He's never been late! There's a certain rhythm to these things: I make trouble, he shows up, we have a few laughs and the game starts all over again!
Or when deprived of his favorite foe....
The Joker: Dear friends... Today is the day that the Clown cried. And he cries not for the passing of one man, but for the death of a dream. The dream that he would someday taste the ultimate victory over his hated enemy. For it was the Batman who made me the happy soul I am today. How I agonized over the perfect way to thank him for that. Perhaps with a cyanide pie in the face. Or an exploding whoopie-cushion playfully planted in the Batmobile. But those dreams were dashed by the weaselly little gunsel sitting there in our midst. The cowardly insignificant gonif who probably got lucky when Batman slipped on the slime trail this loser left behind him. This mound of diseased hyena filth who's not fit to lick the dirt from my spats...! But I digress. The time for sorrow has passed. It's time to look to a future filled with smiles. And I'll be smiling again just as soon as we take that man THERE...and slap him in that box THERE...and roll him into that vat of acid THERE!
Batman makes Joker feel alive in a way that no one else can, and he loves/hates him for it.
Joker's unfulfilled social ambition is his horrific indescribable relationship with Batman.
Or as someone else once pointed out...
Lex Luthor: Does it bother you... bother you at all, really... that Batman likes Catwoman better? HAHAHAHAHAHA He'll never love you, sweetheart! You can keep screwing with his city, but he's never gonna take you to the prom!
In short, curse wise, have him suffer the same sort of sense overloading/deadening effect you see among some worshipers of Slaanesh in Warhammer where after a few years of breaking all taboos and doing every decadent horrific thing imaginable, nothing pings the scale any more and you have trouble feeling excited about anything.... unless of course.... Batman is involved....