Me: An adorable face with liquid brown eyes and velvety ears.
You: Paws with ten digits each, including opposable thumbs,
all bursting with nerve endings.
You see it as a match made in heaven.
But the dog involved might not really enjoy you pawing at her head. And really, would YOU like it if that was how humans greeted each other? (Hey! Not the hair! Don't smear the glasses! Did you wash that hand?) There is a reason the hand shake was invented.
Dogs most certainly don't greet each other by pawing at each others' faces. Heads contain eyes and noses and are very close to throats - heads are to be protected! To meet strangers head to head would be unnerving and also might involve staring - which is terribly rude, as I have said previously. And human hands going for the head might be instead reaching for the collar which no dog really enjoys.
So, if I duck or move away when you reach for my head, please try to see it from my point of view. Once we know each other very well, I love to have my ears rubbed - or my neck scratched right under the collar...yeeees... right there. But this is for family. Don't get up in my grill until we are better acquainted!