One of the great ironies of Guy de Maupassant's short story, The Necklace, is how Madame Loisel actually descends into a lower class of life after her attempt to join high society on the night of the ball. Unhappy with her perfectly acceptable middle class life, she pretended to be something she wasn't in the hope of tasting how the upper class lived. And she did--for a few hours one night. After losing the necklace, however, she and her husband were forced to borrow money, give up their home, and work extra jobs in order to replace the necklace. Instead of achieving a bit of upper class status while living in her middle class world, Madame Loisel fell to even lower depths for a decade while working to pay for the jewels.
Really, this whole story is about social class and the desire to move up in social class.
Madame Loisel is born to a lower class than she thinks she should be in. She feels that it is really important for her to try to move up and mingle with people of a higher social class. That is why she is so eager to go to the ball when the invitation comes.
Madame Loisel is also eager to pretend that she is just as high class as anyone else at the party. That is why she borrows the necklace that causes her so many problems.