I get asked this all time by humans. This human tries to put it into scientific terms:
But he falls into the trap of over-analyzing and stereotyping, like this:
He sees things from the inside out, and in so doing he's developed a very simple theory of "behavior as energy," sort of the E = MC2 of animal consciousness if you will, except it would probably be written as C = ME2, where C is consciousness, M is momentum, and E is emotional energy.
See what I mean? Here, let me spell it out in simple terms: Dogs like to sniff other dogs' butts because they sniff good. Try it for yourself and you'll know.