A really great capture you did there. As you said, the dog really looks a little unsure whether to go or not and the black/white tones underline this scene once more. Not to talk about the background, it fits perfectly as well, gives everything a sad touch that makes you think about what's actually going on in there.
I agree... Thinking of why it might be so, here's a long shot - you think it's possible that subconsciously we consider anything with humans stageable, whereas animals won't pose for a photo and are thus always considered honest in their expressions?
Cheers mate, and thanks for the interesting comment! D.