Chopin describes Mrs. Mallard as "young, with a fair, calm face, whose lines [bespeak] repression and even a certain strength" (Chopin 121). This implies that she has spent a good bit of her life being subjugated, like many women of this time.
When she first learns of her husbands death, she is quite upset. However, after a moment of deep thought, she realizes what his death means for her independence. Now she can act as a self-sufficient individual; no longer does she belong to someone else. She repeatedly whispers "Free! Body and soul free!" to herself, thinking of the "long procession of years to come that would belong to her absolutely" (Chopin 122).
It is obvious why Mrs. Mallard is so shocked when her alleged late husband walks through the door alive and well. However, it is hard to say whether her heart gives out due to a flood of joy or if it is caused by something more. The doctors say that the cause of death was "heart disease....joy that kills" (Chopin 123). This can be interpreted more as shock and devastation that overwhelms her weak heart when her history is taken into account. She is traumatized by the fact that her newfound sense of independence is so terribly short-lived. Thus, it is more likely that sadness is what kills her rather than joy.