The ending is satisfying because the pumpkin wins and Emsworth forgives his son.
An ending is generally considered effective if it is satisfying, and resolves the conflicts of the story. In a humorous story like this one, the ending should also be somewhat funny.
The Earl of Emsworth is “a fluffy-minded and amiable old gentleman.” He is frustrated by his son Freddie, who gets into trouble when in the city and does nothing but mope around in the country. Early in the story the problem that the Earl has is that he spots this son with a girl, who turns out to be related to the Head Gardner, through his new telescope.
[Out] of a small spinney near the end of the meadow there bounded a girl. And Freddie, after a cautious glance over his shoulder, immediately proceeded to fold this female in a warm embrace.
This introduces the story’s main problem. The son is nothing but trouble. How can he be seeing this girl! His son marrying a “sort of cousin” of his head gardener was not what he had in mind. He demands to McAllister, that gardener, that he send the girl away or leave himself. McAllister is offended and says he will leave.
This puts the Earl in a tight spot. He also has a prized pumpkin that the gardener is supposed to be looking after. Because the deputy head gardener isn’t going to cut it. The pumpkin was very, very important to the Earl.
However splendid the family record might appear at first sight! The fact remained that no Earl of Emsworth had ever won a first prize for pumpkins at the Shrewsbury Show For roses, yes. For tulips, true. For spring onions, granted. But not for pumpkins; and Lord Emsworth felt it deeply.
This is why the ending is satisfying. Wodehouse builds up the importance of pumpkin, and how important his son is to him. In the end, his son just hands him a note telling him that he married Aggie. He is more concerned about the pumpkin at this point.
However, the ending is satisfying because it is a happy ending in that Emsworth forgives Frederick and decides to forgive him.
'I may give them a friendly message from you? A forgiving, fatherly message?,
'certainly, certainly, certainly. Inform Frederick that he has my best wishes.'-
'Mention that I shall warch his future progress with considerable interest.,
This is satisfying, because it is a feel-good ending. It also resolves the initial problem because Emsworth is no longer upset with his son, and has decided to support him and give him his “best wishes.”
The other problem is resolved, of course, when the pumpkin wins. So there are happy endings all around.