Mathilde Loisel is unhappy with her life at the beginning of "The Necklace" by Guy de Maupassant because she has been unhappy with her circumstances virtually since she was born.
The first lines of the story explain that Mathilde grew up in a lower class home without any prospects. She never expected much because she never had much, and she understood that her marriage prospects were limited because of that. She marries a man who works for the government, which is perfectly suited to someone of her social status.
While the author uses rather innocuous terms like "simple" and "charming" early on, he eventually gets to the heart of Mathilde's problems: despite being aware of the reality of her low social position, she feels as if she were born to something greater.
Mathilde resents the fact that she married beneath her and she suffers from chronic discontent.
She [Mathilde] 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.
Everything about her life makes Mathilde unhappy. She has a servant girl who comes in to clean her "little house," but all Mathilde can do is regret that she does not have ornate rooms hung with tapestries and silks. When her husband comes home and sits down to dinner, he is perfectly content and even feels blessed by what they have; in contrast, Mathilde is disgruntled and unhappy, imagining luxurious dishes served in rich serving dishes.
The truth is, of course, that Mathilde does not live in abject poverty and deprivation. She has a house, she has a servant, she has perfectly good food to eat, and she has a husband who is content with his wife and his life. The problem is not Mathilde's circumstances; it is her attitude toward her circumstances which creates all her problems.