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

I think the answer is Altruism. Throughout the story, Sergei had been selfless and altruistic. He was presented by a magical fish that can grant him wishes, normally people would wish for themselves but Sergei had used his wish for others. He had wished his sister good health when she was dying from lung cancer. He had longed for a friend and he found it in Yoni, the talking goldfish.