Dogs commonly lick couches when there are crumbs or other enticing smells. If the persistent licking continues without these things present, the cause is usually a response to an obsessive/compulsive behavior.
Obsessive/compulsive behavior is exhibited when the licking behavior creates a trance-like state, increases aggressive behavior and increases the dog's persistence to return to licking after being reprimanded. This behavior is usually linked to a sudden change in the pet's environment, insufficient stimulation and an overall high anxiety level. If these are not factors, the pet should be seen by a professional to rule out parasites and other digestive illnesses.
Increased exercise and socializing can curb a pet's boredom and help to relieve stress. Bringing the pet new toys and introducing it to new environments help increase the dog's curiosity, which decreases a sense of loneliness. If these solutions do not help, it is possible to modify the home to help reduce excessive licking. Furniture can be treated with solutions of vinegar, cayenne or citrus. These treatments are harmless and typically fine with most fabrics. It is important to do a small test sample.
Aromatherapy can also help calm a pet's nerves. Other solutions such as constrictive coating help provide a sense of security and comfort to a dog that may be experiencing a sense of high anxiety and tension.