The moral of "The Fisherman and His Wife" is that a person must be thankful for what he has and not always want more, lest it become impossible for him ever to be satisfied. Those who do not appreciate the small things likely do not have the capacity to appreciate anything and are destined to live a life deprived of joy.
This German fairy tale tells the story of a fisherman who one day caught a fish who begged him to release him because he was really a prince. The fisherman willingly did so, but when his wife heard about it, she made him go back and ask the fish for a nice cottage to replace their shack. Despite not wanting to do so, he did, and the fish granted his request. He was thrilled, but after a week, his wife wanted a palace and demanded he ask the fish for one. He got his request, but his wife was not long satisfied. She sent him back numerous times asking that she be king, then emperor, then pope. More unwilling each time to go back, he consented, and every time his request was granted. Still, his wife found no satisfaction. She wanted to rule the sun and made her husband return to the fish. This time, however, the fish told him to return home to his original shack where he and his wife lived from that day. Because she never found happiness and contentment with anything, she had nothing in the end.