Why does my dog lick metal?

This is one behavior I haven't observed. It falls under the condition of pica - non-food items. It's important to stop the behavior. If the dog can be distracted from the licking and if it really isn't excessive it might be easiest just to live with it. If it really seems excessive, a vet or a veterinary behaviorist may be able to help design a behavioral modification program and medical therapy as needed.