What is the hypothesis of "The Necklace" by Guy De Maupassant?
One educated guess about the story's moral is that an obsession with money and material things can be destructive. Mathilde is frustrated with how her life has turned out. She had envisioned herself living among the wealthier, upper class. Instead of making the best of her lower class life, Mathilde spends lots of time daydreaming about that "better" life.
Her husband is comfortable with his life and he does whatever he can to make Mathilde happy. He sacrifices his own savings to buy her a dress. Mathilde is a hit at the party, but this probably has more to do with her positive attitude than it does with her gown and the necklace. When she loses the necklace, she works incredibly hard to repay the loan. Essentially, she works very hard to pay for something she never needed.
It turns out that the necklace was not that expensive. Mathilde had only thought that the necklace was extremely valuable. She was popular at the party because her behavior reflected her pride in how she looked. This is the other moral of the story. Your happiness in life (upper or lower class) depends upon your attitude. Imagine you are "rich," in any sense of the word, and you will feel rich. Imagine you are happy, and your behavior will reflect that positive outlook.