Matted hair not only dishevels the coat, it actually adds to the dog’s distress and causes skin irritation. When this happens, the dog bites its skin or tries to scratch, causing the mat to grow in size and get even more tangled.
A severely matt dog is not a happy dog. A dog that bites his skin continuously because of the large mats formed will also discolor the hair and leave red stains. The stains are caused by the saliva in his mouth.
Small mats or nodes are easy to remove if the dog is brushed daily or several times a week. Larger knots are formed when part of the dog’s coat has been neglected for some time.
How to Groom a Shih Tzu Very Matted
Source: My Favorite Groomer