Mathilde Loisel has always believed herself destined for things greater than her current circumstances. One quote that demonstrates her selfishness is found in the very beginning of the story:
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.
If you examine the verbs here (suffered is repeated) and the adjectives (poorness, mean, worn, ugly), you can see how Mathilde views her ultimately comfortable surroundings. She is fortunate enough to own a home and to furnish it rather well; yet, she believes that she deserves more. She compares herself to other women and their comparative wealth "torments" her. This demonstrates Mathilde's selfish character early before the conflict even becomes apparent.
When Mathilde tells her husband that she can't attend the party because she doesn't have a dress worthy of such a gathering, he gives her all the money he has stashed back for a summer getaway with his friends. Instead of being thankful, Mathilde demands more:
"What's the matter with you? You've been very odd for the last three days."
"I'm utterly miserable at not having any jewels, not a single stone, to wear," she replied. "I shall look absolutely no one. I would almost rather not go to the party."
The text never states that Mathilde thanks her husband or recognizes his own sacrifice in the efforts to obtain the dress she believes she needs. Instead, she selfishly demands even more. An expensive dress is no longer enough; now she requires expensive jewelry as well.
Luckily for Mathilde, she has a friend with personal access to the kind of jewels she desires, but when she is shown elegant jewelry, nothing meets her standards:
First she saw some bracelets, then a pearl necklace, then a Venetian cross in gold and gems, of exquisite workmanship. She tried the effect of the jewels before the mirror, hesitating, unable to make up her mind to leave them, to give them up.
She kept on asking: "Haven't you anything else?"
Again, Mathilde is presented with a gracious deed, and her reaction is to reach for something even better. At this moment, she has access to pearls, gold, and exquisite gems, but instead of gratitude, Mathilde's selfish heart desires more.
Selfishness, one of Mathilde's character flaws, leads to many years of misery for her when she selects a necklace, believing it to be much more valuable than is actually the case.