Anyone who has ever experienced a Newfoundland or Saint Bernard up close has noticed that things tend to be particularly moist. All dogs drool, of course, but some dogs have a style of mouth that flows more freely.
A dog's saliva lubricates food to help move it down the throat, much like it does in the human mouth. The sub lingual gland runs just below the bottom teeth with openings for oozing saliva from front to back. Keeping supplies of the wet stuff is no problem, in both man and beast there are also salivary glands behind the throat (parotid) and below the cheek (mandibular). Dogs have extra water power with a gland under the eyes (zygomatic).
What makes some dogs's faces seem extra damp is mostly due to lip style. The looser the lips are the more liquid can drip out. With so many breeds of dogs, come variety in head shape, jaw and teeth structure, coat type, etc. So it's no surprize that there are different lip types.
Notice, for instance, the difference between a Bull Terrier's lips and a Bullmastiff's lips. Lips are the fleshy areas that surround the mouth cavity, they tend to be a different color than the other skin. In dogs the lips may be less noticeable because the muzzle surrounds and sometimes over shadows the lips. Some dogs have a hanging muzzle like a Bloodhound, you have to search for the lips. In a stubby muzzle, like a Pug's, the lower lip is obvious but the upper lip in obscured by the muzzle. A Borzoi's smooth tapering style muzzle leaves the lips easy to see.
The skin that hangs from the muzzle around the lips is called the flew. The flew is that flap of skin that is able to fling spittle with a shake of the head.
Size matters too. A Boxer has pretty loose lips but so does Mastiff. And bigger dogs have bigger mouths and bigger glands. The better to soak you with the old head in your lap trick.
Just why do dogs drool? Stay tuned. In the next installment of Drool Theater we'll visit the Pendulum of Yuck.