The narrator suggests that Matihlde’s problem is that she was destined to be wealthy but was accidentally born poor.
Although there is a third person narrator, the narrator focuses mostly on Mathilde. We are to sympathize with her, as the story begins thusly:
The girl was one of those pretty and charming young creatures who sometimes are born, as if by a slip of fate, into a family of clerks.
Mathilde does not accept her station in life. She was born poor, and even though she is pretty it is not enough for her. She even has wealthy friends.
Her husband works for the government and is able to get her an invitation to a ball reserved for the rich. Ironically, she is not pleased with this. She does not feel she can go unless she has a nice dress and good jewels.
It is this inability to be happy with what she has that leads her to disaster, because she borrows the necklace from a wealthy friend, loses it, replaces it, works for years to pay it off, and then finds out the necklace she lost was fake (and she replaced it with a real one).