Why did the Joker lick his lips?

From an interview with Christopher Nolan:

I don't really know where that came from... He actually had a sort of prosthetic appliance for the scars that wrapped over his lower lips, and the first couple of scenes where he was playing it that way, I actually had thought it was a mistake... I thought the prosthetic was coming loose on the bottom lip and he was sticking it down with his tongue or something.

And it was only after we shot a couple of scenes I realized oh no this is a thing he's found, this is a sort of... I think it's a lizard like... or serpent like elemental kind of gesture.. and I think it works... I think it's very creepy...

[Click here to listen to Nolan talk about Ledger's creepy lip-smacking]

The Joker in The Dark Knight had a Glasgow smile. There would have been scars on the insides of his cheeks as well as on the outsides. I thought Heath Ledger was acting out the behavior of someone who had never gotten used to their presence, and was constantly poking and worrying at them with his tongue, especially right at the corners of his mouth.

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.

A movie clip came out a few days ago talking about the joker in the movie, and it is explained why he kept licking his lips.

As we can see in the video below:

At first it was a make-up problem, that when Heath was speaking the prothestics became loosened. So because Heath didn't want to keep gluing the make up to his lips over and over again, he started licking the lips, and then that became part of the character.

Several options come to mind.

1. It maybe a feature of a maniac antisocial. Maniacs have several sorts of behavioral uniqueness. It might be Joker's uniqueness.

2. It maybe an approach like a reptile. Like they do this tongue-work before leaping into a prey, this behavior might indicate Jokers saurian satanic thought process.

3. Like incognito said it might be due to the irritation on his skin caused by his makeup.

4. Might be he does it, when he becomes nervous.

5. A discrete feature proving he is crazy!

My opinion is - Joker wore paint on his face so this might be causing dryness and hence the constant licking