“On the face of it, this doesn’t make sense: Pets take up resources that we would otherwise spend on ourselves or our own progeny. Some pets, it’s true, do work for their owners, or are eventually eaten by them, but many simply live with us, eating the food we give them, interrupting our sleep, dictating our schedules, occasionally soiling the carpet, and giving nothing in return but companionship and often desultory affection.”