Why do Jim and Della feel light and happy as they prepare for supper in "The Gift of the Magi"?

Jim and Della are happy and content before their supper because they have both received the greatest gift of all: the affirmation of unselfish love for one another.
Both Della and Jim have sacrificed their most prized possessions for the other. Indeed, it is a testimony to her deep and genuine love that Della, whose luxurious hair shines "like a cascade of brown waters," is willing to sacrifice her single most-valued possession so she can purchase a watch fob for her beloved Jim. Although her face loses its color when she decides to sell her hair, her eyes are "shining brilliantly" as she excitedly realizes she can procure the gift for Jim that she wants to give him.
Similarly, Jim, who is proud of his heirloom gold watch, is willing to sacrifice his most valued possession in order to purchase a set of beautiful combs for his beloved wife's hair.
Even though Della and Jim are disappointed that the other cannot use the Christmas presents for which they made such sacrifices, they are both blessed with the greatest gift of all: the gift of unselfish love and devotion. Because they are the recipients of such meaningful acts, Jim smiles and places his hands behind his head as he suggests to Della that they put away their gifts.
Certainly, their sacrifices and love express the most valued treasure of all; therefore, they are content as they prepare for supper.