Pitch Black, the king of Nightmare, once threatened to attack again in the burrow of the North. So, that obviously made him take all the Guardians (Bunny, Sandy, Tooth, and Cupid) to a meeting. And… The Man in the Moon chose Jack as the new Guardian.

Jack: "W-Why, why wouldn't he tell me himself?"

Jack sighs in annoyance, looking at the moon.

Jack: "After 300 years, is this your answer? Spend eternity like you, locked in some hiding place, thinking of new ways to bribe children? No, no, that's not for me!"

Jack turns to the rest of the guardians, speaking normally.

Jack: "No offense."