At the beginning of the story "The Necklace," readers meet Mathilde Loisel: a "pretty and charming" girl who was not born with the wealth and distinction that – we're told – her personality and tastes require. She is married off to "a little clerk" husband and lives in a small house. All day long, she glares at her surroundings and day-dreams about the things she wishes she could have:
"vast saloons hung with antique silks, exquisite pieces of furniture supporting priceless ornaments, and small, charming, perfumed rooms, created just for little parties of intimate friends, men who were famous and sought after, whose homage roused every other woman's envious longings."
Clearly, the answer seems to be that Mathilde is discontent because she is poor and wants to be rich. That is absolutely true, but there is more to it than that. The wealth itself is not what is really alluring to Madam Loisel (though it certainly doesn't hurt). Instead, she is obsessed with the life she assumes goes along with such riches and distinctions. If you look back at the end of the above quote, you can see that, in Madam Loisel's mind, the beautiful items merely provide a setting for the thrilling parties full of jealousies and intrigue that she just knows all wealthy people have. She is discontent because she is a middle class woman of the 19th century. She doesn't have to do hard labor to survive, but she also doesn't have a full social calendar and disposable income. She is stuck at home, with nothing to do, no children to care for, no friends to see – it's no wonder she fantasizes about the wealthy life.
At the same time, there is one more reason for Madam Loisel's discontent: she doesn't make an effort to be happy with what she has. Sure, her curtains are ugly and furniture is worn. But she has enough food and a roof over her head and a husband with a decent job; she could be happy with this life. Because she chooses not to be, she seals her own fate.