My dog is pretty affectionate towards her humans - she loves to lick faces, hands, ears, etc. Why do dogs do this? Is this a social thing? Is it some sort of submissive behavior? Does our dog simply like our taste? Is she thanking us for simply being part of her life?
Dogs are a social species and such licking and other signs of affection are what dogs and their wild relatives do as part of relating to each other.
When dogs live with humans, they tend to treat the humans as part of the “pack”, and thus, relate to humans much as they would to other canines. Yes, some of that licking and stuff is submissive behavior, as well trained dogs are taught to view humans as dominant members of the social group.
If you’re the family cook, it might also be because you taste good. Also, in the wild puppies lick at the faces of adult canines when asking to be fed, so draw your own conclusions if it happens around dinner time.
This. It actually stimulates the adult dogs to regurgitate the food they brought home for the pups. When a dog licks your face, it’s actually asking you to vomit on them.
And if my dog has been recently nosing around the litter box for those special crunchy kitty granola dog treats, then I might just comply. 
Our friend’s eight year old dog does this. Lick, lick, lick, lick. It’s annoying, and we don’t pet him because we don’t want to encourage the licking. Why would an older dog do this?
Dogs retain many traits that you only see in juvenile wolves, like barking. This is an example of what is called neotony.
I’m not a dog expert.
Dogs of all ages know their place in the pack and will lick the Alpha dogs face as a sign of submission. Since they usually view humans as the alpha members they will lick you as a sign of submissiveness.
EDIT: What John Mace said. I read somewhere that we basically turned wild dogs into permenant puppies.
He does it to show you that you’re the alpha dog. It’s a sign of respect, like kissing your ring, but less sanitary.
Hm, that’s odd. He’s a very dominant, strong willed dog otherwise. Maybe he’s confused.
If the dog really thinks you’re he alpha, he will come to your side and kneel. This happened to me during a walk along the Pitt Street Bridge in Mt. Pleasant, where many people walk their dogs. A strange, big dog came to me and knelt beside me.
All dogs like to be shown attention and affection. Dogs crave attention and food, and not necessarily in that order. I guess the attention is inbred.
What do you mean by kneel? You mean sit or lie down? I’m having a hard time imagining a dog kneel! 
Dogs do do a sort of “play bow” where they lower their front half. This is an invitation/request to play.
Take a perfectly good wolf, and in only 12,000 years you can turn it into a Boston Terrier. A shame, really. There are several piles of rocks around town that are smarter than our dog.
Obviously he’s tasting you. A dog has to make plans for the future if you die and don’t leave him a can opener for his dog food
My dog is not a licker. She’s 8, and I’ve seen her lick someone’s face maybe 4 times in her life (probably to get food off said face).
Does that mean she considers no one to be her alpha? She’s affectionate otherwise. Just doesn’t want to give kisses.
He sat down by my side and lowered his head.
He wanted you to pet him/scratch him. It was an invitation.
My dog’s a licker and she most definitely is simply showing affection. When I get home from work she follows me around tail-wagging and doing her happy walk until I sit down, at which point she jumps up on me and licks me on the lips for 5 or 10 seconds.
She isn’t being submissive. She isn’t asking for food. She is simply saying “Hey! You’re home! I missed you!” I’m not reading anything more into it than that. I’m one of the pack leaders and she’s genuinely pleased to see me back in the den again.
If he listens to you at all, he probably thinks you’re the alpha. You wouldn’t want a dog who considers himself to be the alpha of his pack.
That’s not submission, that’s “scratch/rub/pet me”.
Dogs lick you in your face to determine if you’ve matured enough and whether or not it’s “that time”. Once they decide you’re ready, they kill you in your sleep and share your carcass with the bad dogs in the neighborhood you always scorned in their presence.
Next question.