The more difficult licking symptoms are often because of psychological causes. In these cases both physical intervention and dog behaviour modification routines are often needed.
A brief example of some of the causes of this problem licking might be due to anxiety. This stress can be anything from separation anxiety, lack of socialisation (as dogs are pack animals and need to be around other dogs. Other causes are lack of exercise, a frightening experience or a major change in their or their owners schedule. The main issue with these types of disorders is that there often is no quick fix. Identify the underlying issue is often the hardest part. Be aware that changing a dogs ingrained behaviour often requires an expert.
There are also several underlying physical internal issues that can cause excessive licking. Some of the potentially fatal diseases are related to intestinal problems (such as related to the liver, pancreas or bowel).
Untreated dental or mouth issues such as ulcers or rotten teeth can be the cause too Central nervous system disorders like brain issues, partial motor seizures
In summary, licking is a very strong part of a dogs natural world for communication and hygiene. When it becomes excessive licking there is always an underlying problem that needs to be treated..