After reading the description of Mathilde at the beginning of Guy de Maupassant's short story "The Necklace", there is little doubt that the character of Madame Loisel is dominated entirely by her ego.
Let's not forget how her behavior is described:
Mathilde suffered ceaselessly, feeling herself born to enjoy all delicacies and all luxuries. She was distressed at the poverty of her dwelling, at the bareness of the walls, at the shabby chairs, the ugliness of the curtains. All those things, of which another woman of her rank would never even have been conscious, tortured her and made her angry.
Here we have a person so deeply entangled with her fantasy life that it is already affecting her overall personality. Mathilde already believes that she IS entitled to a much richer and better life. She even feels tortured, as the excerpt reads, because she does not have all that she wants.
Therefore, her primary motivation to keep the loss of the necklace a secret is basically that, by admitting that she lost it, she would also be admitting to herself that she had to borrow it in order to make it to the ball.
Another possible reason is that she was too proud to go to the rich friend who allowedher to borrow her necklace and tell her that she has lost it. Yet, then again, that would have been a similar type of humiliation to Madame Loisel, as she refuses to accept her reality.
Therefore, it is a comibination of egotism and fantasy what prevents Madame Loisel to openly declare the loss of the necklace that she borrowed from her rich friend.
i think a good example of motivation is that she work for 10 years to pay Madame Forestier her necklace, when she loses it, she feels so bad, that she get motivated to do the right thing and pay the consecuenses.