The character of Madame Loisel is that of a young woman of lesser social status, who comes from a similarly simple family. As a result, she is rendered unable to marry "beyond her station" and find a husband worthy of a large dowry. Therefore, she marries a man of her same status; a clerk who, despite of not having much, is quite content with his life.
The fatal flaw of Madame Loisel is her excessive sense of entitlement. One thing is to want good things for ourselves, and to wish for a better life. Another thing is to hope for the best. However, to detest your life and wish to have another touches on ungratefulness. Madame wanted so much for herself that she despised the little that kept her at least at minimal comfort.
When she has the chance to attend a ball, she begrudges her lack of luxurious clothes and jewels, and her husband proposes that she goes to a rich friend to borrow her jewels. Madame Loisel obviously chooses the most extravagant looking necklace, which shows her state of mind: She wants to dazzle, be showy, extravagant, and call the attention of others.
Sadly, Madame loses the necklace and her pride is so big that she refuses to admit it to her friend. Instead, she and her husband work hard to replace the piece, losing everything they had. When she comes to find out that the necklace was fake, and that she essentially wasted her life over nothing goes to show the irony of her own life. She whined and complained over nothing. She wanted things for her that would have been tantamount to nothing but flash and looks. All of this basically shows that this woman was empty, shallow, and too egotistical for her own good.