We learn, at the end of "The Necklace" by Guy de Maupassant, that the necklace Madame Loisel borrowed from Madame Forestier was a fake. The diamonds sparkled, but they were not real; the same is true of Madame Loisel's dreams to become a member of high society. She had expectations that were not real.
She was not born into a position to expect anything, as the first lines of the story tell us.
She had no marriage portion, no expectations, no means of getting known, understood, loved, and wedded by a man of wealth and distinction; and she let herself be married off to a little clerk in the Ministry of Education.
Neither her social position before marriage or after marriage give her any reason to expect any significant change in her social standing, yet that is exactly what she spends her life wanting. Her life is pretty good (which we learn after she loses the necklace and has to give everything up), but she is full of discontent. When her husband offers her a chance to go to a lovely ball, nothing about the invitation or about getting ready for it pleases her. The fact that she is dissatisfied with her life when it is at its best is a clear indication that she will never be content, that nothing will ever be good enough.
Losing the necklace, which seemed to be a terrible misfortune, turned out to be the thing that saved Mathilde from a life of discontent. When she literally did have nothing, she began to change her thinking and her expectations. Though it was a trying ten years for her and her husband, they will no doubt spend the rest of their lives happier than they ever would have if Mathilde's dreams had ever come true. Her dream of being a member of high society faded quickly once she lost the fake necklace, but she was a better person for it.