I always feel guilty too. I never know whether its worse when he's in the cattery (more of a wrench because its for longer, though at least I know he's safe, well fed, and has medical attention to hand should anything go wrong) or when our friend looks after him at home (I always worry he'll get into a fight, get lost, get beamed up by aliens, etc). He's always fine though, and these days rarely sulks for more than a day at most.