Louise Mallard's sister, Josephine, breaks the news to her gently because Louise is known to have a weak heart. She's scared that the news of her husband's death will cause her to react with anguish and send her to her own death. This is especially true because Louise and Brentley have a good marriage and she loves him. It's fair of Josephine to assume that Louise will react poorly.
Louise eventually goes up to her room and is surprised at her lack of sorrow. Instead, she feels a sense of freedom. Her heart actually feels stronger and more powerful than it did before she got the news. Even though she loved Brentley, his death frees her of their marriage and the necessity of living up to his expectations and fulfilling his wishes.
When Louise comes downstairs and sees that the news was wrong and Brentley is alive, her heart can't take that news. She falls down and dies almost immediately upon seeing him. The doctors who look at her once she's gone say that the joy she felt when she saw Brentley was too much for her heart. No one realizes that seeing him actually took away her strength and newfound freedom—leading to despair and death.