Yuuko's time flow finally starts moving again, after being alive for an unknown amount of years she must depart now since she can't be hold by the world of the living any longer. During her final moments she faces Watanuki, her loyal servant and friend and makes her final wish, "stay alive". So Watanuki made the promise to remain in the store of the wishes until they meet again. Many years later he still keeps that promise giving hope but also taking important belongings from those who ever come to him to make a wish.
[Source: Anime News Network]