Ever since the return of Nightmare Moon, Princess Celestia has noticed that her student and her friends have been getting into a lot of trouble, so, naturally, she worries that they may end up getting hurt while they're unable to protect themselves. Thus, she assigns a very special guard to keep watch over them, without them knowing, of course.
Ever since he'd met the Princess, he'd been serving her faithfully, how could he not? But when she tells him to head down to Ponyville to keep her student and her friends safe, he can't help but wonder if this is the best use of his time.
Unfortunately for Emerald Skies, a job is a job, and he'd be loathe to ignore an order from his Princess.