Question

Why does dog saliva heal wounds

Answer 1

There is no real explanation for why that rumor goes around , but there is a big belief that animals saliva , especially dogs have healing properties for human wounds

Answer 2

Dog Saliva actually doesn't have any healing properties, this is just a myth that is easy to explain. There is a however easier explanation. A dog's tongue is rough which drags pathogens off of a wound, and also debrides damaged tissues of the wound. 

I still wouldn't have a wound licked by a dog though, as the saliva still has a number of bacteria that can cause an infection.