As the story begins,why is Madame Loisel so unhappy with her life

She felt that she deserved the life of the rich but was only able to achieve a humble middle class lifestyle.

.... but she was as unhappy as though she had married beneath her; for women have no caste or class, their beauty, grace, and charm serving them for birth or family, their natural delicacy, their instinctive elegance, their nimbleness of wit, are their only mark of rank, and put the slum girl on a level with the highest lady in the land.

She suffered endlessly, feeling herself born for every delicacy and luxury. She suffered from the poorness of her house, from its mean walls, worn chairs, and ugly curtains. All these things, of which other women of her class would not even have been aware, tormented and insulted her.