They believe her death is caused by happiness at seeing her husband alive. The thing which is ironic about this assumption is Mrs. Mallard is overcome with such great happiness when she finds she is free that when it is taken away, she dies.
"The story of an hour" is based on the theme of freedom and highlights the main character Mrs. Mallard who is not living a happy married life. The story highlights the response when she comes to know about her husband's death.
When Mrs. Mallard comes to know about her husband's death she feels immense joy as from now she will be free and enjoy her life to the fullest.
When she discovered that her husband remained alive and understood that she was still chained by her marriage, that was the point she gave up and passed away.
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