Puppies usually chew on things because they are teething. As new, permanent teeth come in, they can irritate the gum tissue. Chewing on something helps loosen the baby teeth and allows the new teeth to come in. Some dogs continue to chew on things out of habit, and it can be a bad habit if they chew on the wrong things. They can splinter and cut their mouths, or puncture their stomach or intestine if swallowed.
Dogs produce a large amount of saliva.
It is used to help digest their food and also as a cooling mechanism. Dogs don't sweat like we do, so they produce saliva and as they breathe, it helps them cool off. Dogs also produce saliva when they are excited. They don't seem to care where it goes so it often is not swallowed and runs out of their mouths on whatever is close by. Some dogs do love to swim, especially labs, retrievers and other water dogs.