I swear I have seen my dogs get jealous of each other if one gets a walk and the other doesn't. (I have 3, I can't walk them all at once!) Or if one of them get one kinda treat, and another gets a different treat. (2 are big dogs, so they get "big dog" chew bones, but the little one gets "little dog" ones, and he gets mad). Finally proof of my hunch! http://www.npr.org/templates/story/story.php?storyId=97944783&ft=1&f=100 I love my pups.