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. 

In "The Necklace," why was Mathilde unhappy in the early years of her marriage?

Mathilde Loisel is depicted as an unthankful, superficial woman who believes that she deserves to enjoy a life of luxury. Instead, Mathilde Loisel is married to a lowly clerk who works at the Ministry of Public Instruction. Mathilde Loisel desperately dreamed of marrying an aristocrat and suffers from the poverty of her dwelling. She is completely materialistic and is only concerned with owning expensive jewels and fine clothing, which would make other people envious of her. Guy de Maupassant writes,

She [Mathilde] had no dresses, no jewels, nothing. And she loved nothing but that; she felt made for that. She would so have liked to please, to be envied, to be charming, to be sought after (2).

When Mathilde's husband comes home one day and informs her that they have been invited to a ball at the Ministry, Mathilde's initial reaction is to dismiss the invitation because she has nothing pretty to wear and does not own expensive jewelry. Overall, Mathilde Loisel is unhappy in her early years of marriage because she does not enjoy a luxurious life and her husband is not a wealthy man.

The narrator tells us at the beginning of "The Necklace" that Mathlide was very unhappy with her life. What was the reason for her misery?

Mathilde Loisel was a beautiful young woman who felt she always belonged in a higher class than she was in.  She was "trapped" in her class because her parents did not have the money to provide an adequate dowry that would allow her to move into a higher class, and so therefore she hired a clerk who worked in an office.  In truth, the Loisels weren't exactly poor.  They had a servant, they lived in a fairly large house, but it wasn't how Madame Loisel dreamed her life would be.  Madame Loisel was tortured the fact that she lived an ordinary life, and even during dinner she dreams of how her life should be.  This is a criticism of materialism and the class system because Madame Loisel didn't appreciate what she DID have.  Her loving husband even manages to get an invitation for a prestigious ball, and this only upsets her more since she obviously doesn't have the clothes or jewelry to attend.  This trapped feeling of misery will be her downfall throughout the story.  

The narrator tells us at the beginning of "The Necklace" that Mathlide was very unhappy with her life. What was the reason for her misery?

Maupassant makes it clear that Mathilde comes from a relatively poor family but happens to be an exceptionally beautiful girl who could have made a much better marriage if she had had better opportunities. She married a humble office worker who could not provide the pleasures and luxuries she saw other young women enjoying. She was not able to attend the kinds of social functions she could only read about in the newspapers. She was unable to show off her beauty, grace, and charm because of their lack of money and inferior social position. Her life was confined largely to cooking and housework. She felt herself getting older and losing the glow of her youthful beauty. Her husband was a virtual nobody without any great future in the government because he had no important family connections and did not seem to be especially talented or ambitious.