In this story, the Forest King is doubtful about granting Mr. Peters three wishes. He believes that all humans waste their wishes, which makes him think that Mr. Peters will be no different. However, he accepts anyway and gives him his wishes.
We would normally expect Mr. Peters to use his wishes to make himself happy. And although he does this at first, he eventually uses his wishes to bring his wife happiness when he turns her back into a swan. This shows that Mr. Peters proved the King wrong. Although he still had a wish available, and although he could have used this to make himself happy, he ends up using his wish to make his wife happy. This shows that he puts his wishes to good use. Moreover, the swans believe so too, and they are devastated when they hear about Mr. Peters' death at the end of the story.
