I guess domestication has encouraged dogs to shed some of the inhibitions they must have had as wild animals. Now they feel free to behavior in ways that are… well, inexplicable. What in the world are they doing? Why keep trying to sit on the other dog’s head? And of course there’s always “Oh, boy–dark, soupy mud! Why don’t I wallow in it, and then go home and jump up onto the sofa?”
It passeth human understanding.