Why do dogs like to be petted? And seem to like or need that type of affection?
One thing we don't have to be taught is what is pleasurable and what is not and that may be all that is. To say that it is the affection they feel and reciprocate falls again under the heading of believing we know what they think and how they feel.
I don't disagree but I'm trying not to include evidence that I can't verify visually with some certainty.