Let us remember what happens to Edna Pontellier during her time on the beach. She experiences a kind of "awakening" that makes her aware of the emptiness of her own life and marriage and also give rise to a desperate desire to be loved and to be held that she believes can be satisfied through her relationship with Robert. However, as Chapter Fifteen makes clear, it is ironic that having undergone this internal transformation, Edna should have the means of gaining what she feels she desires snatched away from her so cruelly through Robert's voyage to Mexico and away from her. Note how her thoughts are described at the end of this chapter:
The present alone was significant; was hers, to torture her as it was doing then with the biting conviction that she had lost that which she had held, that she had been denied that which her impassioned, newly awakened being demanded.
Thus we can see that one reason for her unhappiness comes from her new awakening, with its new desires and wants, and the removal from her of the person who can satisfy those desires and wants. This results in Edna being left a deeply unsatisfied and unhappy woman.