Many people believe that their dog's mouth is so clean that they will let them lick them all over. There really is no need for a scientific comparison, just take some time to observe a dog. Are you familiar with what your dog does with it's mouth? Well, many dogs like to smell and lick things that interests them, especially with things outside the house. They go through trash, socialize with other dogs, and their private parts. From all the activities listed, dogs pick up millions of germs, and you definitely don't want all that over your face!
This theory came about because we see dogs lick their wounds. Therefore, some people assume that dog saliva is clean and possibly holds healing properties. This is very untrue. We share as much bacteria in our mouths as dogs. The only difference is that dogs have different germs. Some of these germs are only found in dogs and can't be passed to humans, other germs can spread from dog to human. Dogs lick their wounds because they have a rough tongue that cleans away dirt in the wound thus, making it heal faster.
My advice to dog owners is to keep your dog away from your face, but cuddle up and give him losts of love. Just don't do it mouth to mouth.