Why did the Joker lick his lips?

It was actually an honest bad habit of Ledger's. He would often lick his lips and it fit in with the character. The joker is symbolically described as a dog in the film and there is many references.

1. He is a self proclaimed "dog chasing cars"


2. He is the unloyal hungry dog as compared to Chechen, the Russian's, pets/protection


3. There is a scene where he is hanging out the window of a GCPD patrol cruiser and licking his lips as wind blows his hair much like a dog would.


4. He is often seen licking his lips.

But, it began as just Heath's bad habit and personality

The symbolism is he is an animal that can play tame but, much, like a pitbull, he is raw power and wilderness.