Why was the story "The Necklace" written?In other words, what inspired the author to write "The Necklace"?

Unless we can really get on our hands on what might be called 'external evidence" -- evidence such as the early manuscripts and notes, letters, and content of conversations with friends -- we can never really know what an author's intentions are. Two New Critics (Wimsatt and Beardsley) famously make this argument in their essay "The Intentional Fallacy."

New Criticism is not the only way to read literature, but the argument of the "intentional fallacy" is solid. As I see it, based on but not limited to Wimsatt and Beardsley's argument, what we normally have in front of us is the text, not the author; we can talk all day long about what the text does, how it is structured, how it shapes the reader's response in certain ways, and so on. If we read well and closely, we can ground our discussions in evidence drawn from the text. When we move from the text to the author's intentions without also bringing in external evidence, however, we are simply projecting our own views of what the text means onto the author. Even if we can call up the author and ask them, as a friend might, what they had in mind when they wrote a certain story, we are not likely to get the answer that we are looking for. Artists often do not analyze their own writing, do not give fully honest accounts, do not fully understand what was happening during the creative process, and so on.

The author, to return to the language of Wimsatt and Beardsley's argument, is not the "oracle"; we must arrive at the meaning of the text on our own.