Why was Mathilde unhappy despite being pretty and charming?

Mathilde Loisel, the main character in Guy de Maupassant's short story "The Necklace," is unhappy because she longs for quite a different life than the one she is living. Although she is a "pretty, charming young woman," she is a victim of the rigid class system in nineteenth century France. Because she "had no dowry," the best she can do is marry "a minor civil servant," who, although seemingly a fine man, cannot provide her with the wealthy lifestyle she craves. She daydreams about "thick carpeted reception rooms with Oriental hangings, lighted by bronze torches" and "great drawing rooms upholstered in old silks, with fragile little tables holding priceless knickknacks."
Instead, Mathilde lives in a modest apartment with "shabby walls" and "worn furniture" and she has dinner each night with her husband at a "round table with its three-day old cloth." Moreover, Mathilde has "no proper wardrobe" or jewelry. And, of course, these are the things she most prizes in life. This class envy causes Mathilde and her husband all sorts of problems and they eventually lose their modest, yet comfortable life (after all, they had a maid) and sink into poverty in order to pay for the supposedly expensive necklace Mathilde loses on the one night when she actually ascends into the upper class.
In fact, for one night, Mathilde is the belle of the ball, dancing and being admired: "She was the prettiest woman there—resplendent, graceful, beaming and deliriously happy." Unfortunately, like many things in life, Mathilde's joy is fleeting, and her carelessness in losing the necklace changes her life forever. Amazingly, however, de Maupassant seems to suggest Mathilde ultimately survives (she helps pay off the debt with "unexpected fortitude") and is possibly a better person at the end of the story despite discovering that all of her "privations" were in vain because the necklace had been a fake.