I want to write an argumentative essay about this quote from "The Story of an Hour": "When the doctors came they said she had died of heart disease...

This quotation, the last line of the story, refers to the first line of the story and the fact that Louise Mallard is believed to have heart trouble.  Because of this malady, her sister tries to gently break the news that Mr. Mallard, Louise's husband, was killed in a train accident.  Louise does not react the way her sister or we would expect her to.  Rather than mourn the loss of her husband, she actually...

This quotation, the last line of the story, refers to the first line of the story and the fact that Louise Mallard is believed to have heart trouble.  Because of this malady, her sister tries to gently break the news that Mr. Mallard, Louise's husband, was killed in a train accident.  Louise does not react the way her sister or we would expect her to.  Rather than mourn the loss of her husband, she actually seems to rejoice that she is now "'free, free, free!'"  She sits at a window, suddenly aware of all of the evidence of "new spring life" including the "delicious" rain, the "twittering" sparrows, and the "blue sky": positive and vibrant images that match her new life as a free, single woman.

Watching her reaction to this news, there are clues that Louise's heart is actually not fragile.  The narrator says that the lines in her face "bespoke repression and even a certain strength."  Further, "Her pulses beat fast, and the coursing blood warmed and relaxed every inch of her body."  She doesn't sound weak, and her excitement doesn't enfeeble or enervate her; instead, it warms and soothes her.  It seems, perhaps, that the only reason her heart might have seemed weak before was due to the "repression" she has apparently endured as a result of her marriage.  

Her husband was not a bad one, and she's aware that he loved her, but she is happy that "a long procession of years to come [...] would belong to her absolutely."  She now prays that life will be long when, only yesterday, "she had thought with a shudder that life might be long."  It was, apparently, her life as a wife, as one who must compromise and submit, that weakened her; now, though, her new independence gives her strength and vigor that she lacked when her husband lived.  Thus, when her husband opens the door, everyone else assumes that she died of a joyful shock that was too much for her heart.  Ironically, it was the opposite: she died of the terrible shock that the freedom she believed was now hers was snatched away.