What theme about happiness does this story convey through sergei's situation and actions and the outcome of events

"What, of This Goldfish, Would You Wish?" conveys an important message about being happy through Sergei's situation and that is "Being Selfless" and put other people' s joy before yours. Sergei had three wishes and all of them for others even when it costed him to let go of his fish, he chose others. That is the theme of Happiness.