Mathilde is born into a poor family, but she has always felt she belongs to the upper class. She is not proud of her income level or her social class. But she prides herself on believing that she is better than her social and economic state. 

Mr. Loisel comes home one day "with a triumphant air" because he and his wife have been invited to a ball held by the Minister of Public Instruction. He worked hard to get this invitation so his pride is justifiable. His wife is dismayed because she doesn't want to attend such an event unless she can wear expensive clothing and jewelry. He gives her money for a dress and she borrows a diamond necklace from Mme. Forestier. When she goes to the ball, she has a wonderful time because she is wearing expensive things. Her attitude changes solely because she feels like part of the upper class. This is her proudest moment: 

She danced with intoxication, with passion, made drunk by pleasure, forgetting all in the triumph of her beauty, in the glory of her success in a sort of cloud of happiness composed of all this homage, of all this adoration, of all these awakened desires, and of that sense of complete victory which is so sweet to woman's heart. 

Her pride is largely superficial. She is proud to be admired and adored. She is proud to be wearing the diamond necklace. But it might be her change in attitude that has made people admire her. Her pride is short-lived because she loses the necklace. In order to pay back the debt from replacing the necklace, Mathilde learns what hard work is. She learns what it is like to be truly poor. She still dreams of the ball and living a life of luxury, but there is a subtle indication that she takes pride in repaying the debt. In repaying the debt, she plays her part with "heroism." She embraces the challenge.