In Batman why does Joker keep saying he and Batman need each other?

In Arkham City he says it and in Heath Ledger's protrayal of the Joker. And maybe Batman the animated series. What dies that mean that he needs batman and batman needs joker?In Batman why does Joker keep saying he and Batman need each other?
The Joker: Oh, you. You just couldn't let me go, could you? This is what happens when an unstoppable force meets an immovable object. You truly are incorruptible, aren't you? You won't kill me out of some misplaced sense of self-righteousness. And I won't kill you because you're just too much fun. I think you and I are destined to do this forever.



Batman: You'll be in a padded cell forever.



The Joker: Maybe we can share one. You know, they'll be doubling up, the rate this city's inhabitants are losing their minds.





As mentioned in The Dark Knight , the Batman and the Joker are two sides of the same coin . One devoting his remaining sanity and capability to ridding his city of evil , and another reigning it down onto the streets with no apparent cause or motive. Batman does what he does to make sure that people like the Joker are off the streets and hurting no one , while the joker does what he does simply to prove the point that chaos rules all , and underneath the polite day to day boundaries society has risen for us , when stripped bare we are all evil , by human instinct . They both need the other to survive as Batman to him is the ultimate force of justice , the proof that underneath it all man is good , man is better then what is perceived .In Batman why does Joker keep saying he and Batman need each other?
I don't know for sure but it could be Yin and Yang... as in there can't be any darkness/evil without light/good and vice versa.



Without no villain there's not hero in way.



Hope this helps!In Batman why does Joker keep saying he and Batman need each other?
The Joker is different from other criminals because he isn't after money or power, he just wants to cause problems. He needs batman because seeing Batman trying to stop him is one of the main things he likes doing. Batman need's him because other criminals who are after material things can be stopped by the police. But since Joker isn't after any concrete reward, they can't deal with him.