The reasons why dogs drool while sleeping:

1. It may be that the dog’s mouth is injured or the mandible is fractured, resulting in increased drooling.

2. Dogs are born to drool. Generally, large dogs drool more than small dogs. This is genetically determined.

3. It may be caused by the dog’s teeth becoming loose at that time.

4. It may be that the weather is hot and the dog can only use its toe meat and tongue to dissipate heat.

5. It may be caused by the aging of some dogs, which causes the muscles in the dog's mouth to become loose and droopy.

6. It may also be a normal conditioned reflex when the dog is sleeping and dreaming. The conditioned reflex causes the dog's salivary glands to secrete vigorously and increase saliva.