Why does Madame Loisel select the necklace rather than any other piece of 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...

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. 

All of a sudden she discovered, in a black satin box, a superb necklace of diamonds; and her heart began to beat with an immoderate desire. Her hands trembled as she took it. She fastened it around her throat, outside her high-necked dress, and remained lost in ecstasy at the sight of herself.

Ultimately, it is Madame Loisel's desire for a higher status and wealth and dissatisfaction with her life that contribute to her borrowing the jewelry.

Madame Loisel borrows the necklace and unfortunately loses it. This event is a turning point in her life because she must work tirelessly to afford a replacement necklace. As a result, Madame Loisel is reduced to a lower status (both economically and socially) than she was prior to borrowing the necklace. In the end, we learn that the necklace she coveted and which symbolized wealth to her was indeed a fake and not worth much more than fresh flowers.