The author introduces us to the central theme of the story from the very first line of the book. He says of Mme. Loisel,
"She was one of those pretty and charming girls who are sometimes, as if by a mistake of destiny, born in a family of clerks".
This line tells us that Mme. Loisel is not going to be happy with her life. She wants to be rich, but she will live a simple life instead, and be very unsatisfied with it.
Mme. Loisel's husband does not share her dissatisfaction with life, as he shows at dinner when he uncovers the soup tureen and declares, with an "enchanted" air,
"Ah, the good pot-au-feu! I don't know anything better than that!"
The life he lives with Mme. Loisel is quite enough for him; he is satisfied with what he has.
When the elegant ball which Mme. Loisel had so wanted to attend is over, the author says,
"All was ended, for her".
This short statement is ironic in that, yes, the ball is over, but so is Mme. Loisel's life as she knows it. She has never been satisfied with her situation, but now that she has lost the necklace, even what she has is about to be taken away.
The jeweler whose name is on the box holding the necklace Mme. Loisel had borrowed from Mms. Forestier gives us a clue that the necklace might not have been what it had seemed. He tells the Loisels,
"It was not I, madame, who sold that necklace; I must simply have furnished the case".
The Loisels, undaunted, continue on from jeweler to jeweler, never considering that the case might have held a necklace which had not been bought at a fancy place. They are unable to distinguish the case from its contents, or what is inside from its outer appearance.
When Mme. Loisel reveals to Mme. Forestier that she and her husband had replaced the original necklace, Mme. Loisel
"...smile(s) with a joy which (is) proud and naive at once".
This statement sums up Mme. Loisel's whole problem. She is proud, and wants to have nice things so that she can show them off, but she is naive, actually in two ways. First of all, she does not recognize genuine quality when she sees it, as in the case of the necklace, and second, she does not understand that a person's value does not come from what they have; that is, their exterior appearances and possessions.