Dogs lick their noses to carry scent chemicals from the outside of their noses to the olfactory centers in the roofs of their mouths. Licking the nose is a way of detecting fainter smells. In this process, of course, your dog's nose gets wet. Dogs don't lick their noses when they aren't sniffing for something out of the ordinary. A dry nose doesn't mean your dog is sick, just that your dog isn't on the trail of something.