In the competition between him and Gabriel to see who wins the title of favorite uncle, Crowley's convinced he has it in the bag, one trick up his sleeve that the trickster himself can't top, and that's that Jack adores hellhounds. Juliet goads Jack into playing a game of fetch that lasts hours, the unstoppable force of toddler energy meeting the immovable object of a hellhound's unending persistence.
Then, it turns out Gabriel does have a dog of his own, and without his ace in the hole, Crowley has to be twice as clever (and take Jack to the zoo twice as often) in order to win.