Madame Loisel marries a clerk because she is not from a wealthy family and therefore cannot marry a wealthy man.
She had no dowry, no expectations, no way of being known, understood, loved, married by any rich and distinguished man; so she let herself be married to a little clerk of the Ministry of Public Instruction. (p. 1)
Madame Loisel is beautiful, and for some reason feels that she deserves better. Even though she does not come from a wealthy family, she feels that she deserves to be wealthy and to have the luxuries wealth provides. So she resents her husband and her household, because it is not want she wants. This ultimately leads to her downfall, when she borrows the necklace and lets loose a series of unfortunate events that ends with her losing her youth and beauty as well