why do with have a constant flow of saliva in our mouth?

Answer 1

To keep our tongue and the inside of our cheeks from drying out, not to mention digesting stuff.

Answer 2

So we can chew food and break it down to swallow and so that we can talk, I read this book and this guy had to bite his arm and like drink his blood (only a little) because he was so parched just to say help!.