In Mark Twain's "The Celebrated Jumping Frog of Calaveras County," what prompts Simon Wheeler to tell the story of Jim Smiley?

The humor of this story lies in the answer to this question, so it’s a very important one. First, it’s important to understand the overall narrative structure of this short story. It’s a frame narrative, or a story within a story. The first narrator, who is unnamed (though has a voice similar to the author, Mark Twain), tells the story of the time his friend tricked him into listening to Simon Wheeler tell a long, mythical story about someone named Jim Smiley. At the start of the main story, the narrator writes that he went to Simon Wheeler “at the request of a good friend of [his]” to ask about someone named Leonidas W. Smiley. After reflecting on the incident, he realizes his friend’s true intentions:

I have a lurking suspicion that Leonidas W. Smiley is a myth; that my friend never knew such a personage; and that he only conjectured that, if I asked old Wheeler about him, it would remind him of his infamous Jim Smiley, and he would go to work and bore me nearly to death with some infernal reminiscence of him as long and tedious as it should be useless to me.

In other words, the narrator’s friend sent him to ask about someone named Leonidas W. Smiley because he knew it would remind Wheeler about Jim Smiley, and the friend likely knew the story Wheeler would tell as a result. It’s all just a big practical joke.

The cynical, superior voice of the narrator contrasts greatly with the exaggerated southern voice of Simon Wheeler. It is likely that the aforementioned friend was aware of this feeling of superiority in the narrator, and thus knew that the narrator would find the story “monotonous” and “interminable,” or endless. The whole incident results from a joke played on the narrator, which the narrator doesn’t seem to find very funny. The reader, though, finds humor in the story itself and in the narrator’s obvious discomfort.