Why is Mme. Loisel upset at receiving the invitation to a fancy party?

Mme. Loisel is upset at receiving the invitation to a fancy party because she feels inadequate and unworthy of such an event. She envies the wealth and luxury of others and is frustrated by her own modest lifestyle. Instead of seeing the invitation as an opportunity, she focuses on her lack of beautiful clothing and jewelry, which heightens her feelings of discontent and disappointment.