@asp402016,
My dog will do that too, often putting her head over both my nose and mouth when I am in bed. We kid around about it and call it "smother love", but it can get really annoying, particularly when it makes it hard to breath.
It's clearly an affectionate thing with my dog, it's as if she wants to get as physically close to your face as possible. Sometimes it also seems protective, like she wants you under her to keep you safe. When she resists having her head moved off my face, it also seems like dominance, because she will get very determined about keeping it there.
Why do dogs do this? I'm not really sure. Puppies in a litter will sleep on top of each other. I also once read somewhere that, when a dog puts their throat across your mouth, it indicates complete trust, because the throat is the most vulnerable part of the dog's body and they are willingly exposing it to your mouth. How true that is, I have no idea. I've asked my dog, but she just shrugs her shoulders.