The question "The Story of An Hour" seems to ask is "Why did Mrs. Mallard die?"
I see this question in the story because they mentioned in the beginning and in the end of her "weak heart" and she passes away in the end by "the joy that kills." Ironcally, she was devastated by her husband's return, which ended her freedom from their marriage. Mrs. Mallard was young, so it is possible that her condition was metaphorical for her emotional state.
Kate Chopin might answer with question by saying that Mrs. Mallard passed away because she realized her need for independence was greater than her love for her husband.
