Why is Mme Loisel unhappy when she receives an invitation to an evening reception in "The Necklace"?

The simple
answer is that Madame Loisel is upset that she does not have anything to wear that is worthy of
the occasion. More noteworthy still is how she reacts to the invitation, showing complete
contempt and disregard for the efforts of her husband to secure the invitation for
her. 

Instead of being delighted, as her husband had
hoped, she threw the invitation on the table crossly, muttering:

"What
do you wish me to do with that?"

There is much more
to Mathilde's reaction than just the fact that she has nothing to wear.  Mathilde is a
self-absorbed woman who feels that she is entitled to all the good things in life, despite of
the fact that she has done nothing to earn them, nor has she ever come from money, in the first
place. 

When she says that she has nothing to wear, she does not say this
with the humbleness that distinguishes a good woman of limited means. Instead, she uses sarcasm
and haughtiness, as if demanding from her husband--or from life itself--that she gets those
things. 

She even goes as far as telling her husband that 400 francs would do
for a new dress (which she is only going to wear once), so she took the savings that he had put
aside for a hunting trip to buy the dress. More importantly for Mathilde was the jewelry to
wear. Since she had none, she had to resort to her rich friend, Madame Forestier, who lends her
the necklace for which the story is titled: a necklace that gets lost, and whose replacement
will cost Mathilde her entire life.