Shimmer is a stress licker. This type of licking is not submissive, or affectionate, but more compulsive. It consists of stereotyped licks, with repeated head movements. When she's feeling anxious or stressed, she'll repeatedly try to lick you - your hands, neck, face, shirt....there is no actual relation to you at all, you can see in her eyes that she's almost not even focused on the licking but almost lost inside her own mind somewhere. When she's not feeling anxious, though, her kisses are those of a normal dog, a quick lick here or there, usually only when asked for it.
Gaci will give kisses on cue. I know they aren't "kisses", but they are affectionate for her.
I have an idea that the majority of dogs stop licking out of "deference" (that is, what a puppy would do to its dam) when they get out of juvenile puppyhood. Most dogs, as adults, don't lick each other's mouths ritually like they do when they are puppies to their dams. With my guys, they will lick beards after a meal, but other than that never lick mouths in that manner. And working at the boarding kennel, and being around a lot of dogs over the years, I see very few dogs ritually lick the mouth of another dog. They will, however lick ears and groom each other. But I don't see it as anything to do with deference, since in any one sitting the dogs may groom each other, or they'll take turns, and there is no 'order' involved. To me it appears to be simply affection.
The dogs that I have seen lick mouths as adults tend to be either one or a combination of the following - sensitive, soft dogs, neotinized breeds sometimes do it more than the more primitive breeds do. Dogs that really care what other dogs think of them, and often have other body language showing that - crawling up to the other dog, rolling over, ears back and body lowered, etc. Usually the licking, is done in a highly ritualized manner. I've also seen it occur in highly anxious dogs that are unsure of how to otherwise act, I've seen it in puppies that were improperly weaned in litters. I have rarely seen it in stable, confident, adults, and I seen differing levels of tolerance by the lick-ee of the lick-er.
I think in terms of dogs licking humans, it's not about deference but rather what is learned as a puppy (or when you get the dog, but more likely with puppies). Puppies are often reinforced a lot for giving "kisses", by most people who meet the puppy. So the puppy likely learns, more classically than operantly, that kisses are a great thing to do and often result in fun things happening. The more experience a puppy has with licking people, the stronger the behaviour will become, if the behaviour is reinforced (simply smiling can reinfroce it, or talking to the dog). So it becomes habitual. The body language involved in kissing people is rarely what you would see in a dog licking the mouth of another dog, which also seems to partially indicate that there is more learning behind it than instinct. Also, puppies that aren't raised much around people, tend to be less "kissy" than those pups raised with a lot of people. It's One other thing I have noticed, is that sometimes dogs that know each other will licky-face after reuniting after a time apart. Again, it's not ritualistic, and the body language can vary depending on the dog (no consistency with "submission";). It seems that these dogs are simply doing a doggy-style of a happy greeting after being apart, much like how two humans will hug or shake hands after having been apart for a period of time. not an absolute, as everything is dog specific, but these are things I have noticed over time.
I think licking, with humans, is not about deference, but more affectionate, and even for some dogs I would wager that there is very little affection involved, but rather it's simply habitual and learned, much like a "high five". Licking also releases those feel-good chemicals in the brain, so that could be a part of it as well (you tend to see this more in dogs that suck/lick inanimate objects or itself, and its seen more in dogs with some behaviour issues, but it might play a role).