Licking is not always a sign of affection, but mostly it is. When dogs lick each other or humans, this is a release of pleasurable endorphins. It shows that they are comfortable and relaxed. You may notice the timing they do this is often when they are relaxing with their owner on the sofa while watching tv, or if you stop in the kitchen to pet them for a little while. In these cases, it will be mostly that they are displaying signs of affection.