Drooling is a natural response in dogs when they are expecting food. Drooling readies the dog to eat by lubricating its mouth. When a dog knows that dinner time is close, drool production kicks into high gear.
Drooling can be triggered by something that gets caught in a dog's mouth. For example, he might chew a stick or bone and get a splint caught in his mouth. If a bee, spider, ant or other insect stings or bites the dog, the area may swell and the dog may drool excessively. A cracked tooth and other dental problems such as infected gums can also cause drooling.
Certain digestive problems can make a dog drool. This is especially common if she is having an allergic reaction to food or if she has eaten poison. If you have reason to suspect these causes, you should take the dog to a vet immediately. Heat stroke, epilepsy and even certain heart conditions can also have drooling as a symptom. Drooling to the point of foaming is one of the characteristics of rabies.
A dog may drool when he is excited or in a stressful situation. When the animal is overstimulated by being around new people or other dogs, his drool production often increases in response to the stressor.
Some dogs are more prone to drool simply because of their physiology. Drooling is much more noticeable in breeds with loose lips. According to the Dog Breed Information center, dog types known for their excess drooling include the Great Pyrenees, Bulldogs, Saint Bernards, Great Danes, Mastiffs and Shar-Peis.