The licking is friendly submission. My dog does it all the time, much like a puppy would to an older dog. The approach is usually from the side (more polite, less challenging than a direct approach). Getting down low and licking from underneath is another, stronger, sign of submission. My dog will still do that with dogs that are a little pushier than she is. But generally it's a toss-up what she'll go for first - butt or face! Depends entirely on the vibes she's getting from the other dog.