Why does Madame Loisel choose the necklace in "The Necklace"?

Madame Mathilde Loisel selects what she thinks is a diamond necklace because she believes it is "superb" and just the kind of dazzling jewelry that will enable her to attract the attention of the male guests and make her appear affluent.

Mathilde Loisel, who has been born into working class family, "grieves incessantly" because she feels that she was born for "all the little niceties and luxuries of living." She has long dreamed of being a guest at fashionable dinner parties in rooms decorated with tapestries that have images of historical characters and fairyland forests. There, also, are tables of gleaming silverware, and delicious dishes served on beautiful china.

One evening as Mme. Loisel dines with her husband, who is a minor clerk in the Ministry of Education. As they sit at a table that has been covered with the same cloth for three days, her husband hands her an invitation to an evening reception at the Ministerial Mansion. Rather than expressing delight, Mme. Loisel tosses the invitation onto the table, complaining in her self-pity, "What good is that to me?" Her husband, who does not understand why she has reacted in this manner, tells her that he has thought she would be thrilled, adding that he has had a difficult time procuring this invitation. But, the self-centered Mme. Loisel complains that she has no dress to wear. So, in his generosity, her husband sacrifices money he has saved for himself so that his wife can purchase a lovely dress for the reception.

But a pretty dress is not enough. Mme. Loisel feels that she needs some jewelry because 

"There's nothing more humiliating than to look poor among a lot of rich women."

After remarking upon his wife's silliness, her husband suggests that she visit an old school friend, Mme. Forestier, and ask her if she will lend her some jewelry. With a cry of joy, Mathilde Loisel exclaims, "Why, that's so! I hadn't thought of it." Later, at the home of Mme. Forestier, Madame Loisel discovers what she thinks is "a superb diamond necklace" in a black satin box; she holds it to her throat with trembling hands while she looks at her reflection "in ecstasy." With hesitation, Mme. Loisel asks if she may borrow the lovely necklace, and with delight she hears, "Why, of course." She thanks her friend as she hugs her.

At the party, Mme. Loisel, wearing the dazzling necklace, is the "prettiest one there, fashionable, gracious, smiling, and wild with joy." All the Cabinet officials ask her to waltz with them. Even the minister notices her. She is in ecstasy as she gains admiring glances and achieves "complete victory." Mme. Loisel's actions demonstrate her superficiality and her shallow emotions. Thus, her nature is much like the necklace she has chosen because it, too, is false, superficially lovely, and transitory in its beauty. 

Why does Madame Loisel choose the necklace over other jewelry?

Madame Loisel's character is superficial and concerned about status. When invited to the Minister of Public Instruction's soiree, a difficult invitation to receive considering her husband's status as clerk, she is not satisfied. "She looked at [her husband] with an irritated eye, and she said, impatiently: 'And what do you want me to put on my back?'" Her initial reaction is not of gratitude but of ingratitude because she doesn't have a dress to wear to the event. 

Madame Loisel does not care that her husband may not be able to afford finer items such as dresses or jewelry; she only cares that she presents the illusion that she is wealthy and of an appropriate status. This is what leads her to borrow jewelry.

Madame Loisel chooses the necklace rather than wearing flowers or even another piece of jewelry because the necklace symbolizes status and wealth. 

In "The Necklace," why does Madame Loisel choose the diamond necklace to borrow?

Madame Mathilde Loisel chooses the diamond necklace because she wants her one glittering night out to be perfect; to her, perfection is luxury, beauty, and ease. When her friend allows Mathilde to look through her box of jewelry, Mathilde sees the "superb" necklace and

her heart began to beat covetously. Her hands trembled as she lifted it. She fastened it round her neck, upon her high dress, and remained in ecstasy at sight of herself.

The diamond necklace makes her feel an intense desire to own it; it makes her feel ecstatic to see herself in it. It must be incredibly beautiful and incredibly expensive-looking, because she can only ask to borrow it with a good degree of "hesitation" and "anguish." Mathilde wants to stand out, to be the most lovely woman in the room, and so she chooses the necklace, then, out of vanity and pride. She has always felt that she was "born for every delicacy and luxury," and now—wearing this necklace—she gets to feel, for a night, as though she really has that life.