The story-external answer is that Tolkien is foreshadowing the surprise
ending. Tolkien loves foreshadowing, and does it often.
The story internal answer is that Sam was very much in love with fairy tales
that traditionally ended "and they all lived happily ever after." Like
you, and I suspect like Tolkien, Sam hated to read "The End" and wanted to know
more.
“I dislike Allegory - the conscious and intentional allegory - yet any attempt to explain the purport of myth or fairytale must use allegorical language. (And, of course, the more 'life' a story has the more readily will it be susceptible of allegorical interpretations: while the better a deliberate allegory is made the more nearly will it be acceptable just as a story.)” (From Tolkien Letter # 131.)
