His motivation is quite simple, really. The Joker is just a simple man, trying to prove a point.

He doesn’t do it for the money, he doesn’t do it just for the shits and giggles (despite what he might want you to believe), and he doesn’t “just like chaos”. He does it to prove a point: That every man can break his moral code.

He wants to prove that civilization is nothing more than a word on paper. He wants to show everyone that the best way to live is without rules and without morals, without a plan. He wants to show us all that we can be just as crazy as he is. It’s like the great Heath Ledger’s Joker said to Harvey Dent:

It’s on that note we can deduce his other great motivation:

Batman.

The Joker wants to prove that everyone can break their moral code, and what better way to do that than make the very symbol of law and order break his moral code? It’s why he’s chosen Batman as his nemesis, because he knows that if he ever makes Batman kill (which is his moral code) anyone, then he has won the battle. Even if he’s one who ends up being killed.