When the doctors came they said she had died of heart disease – of joy that kills. (23)
Based on the other characters' views of events, this interpretation of Mrs. Mallard's death seems understandable, if tragically so. It's unfortunate but natural that after such an exhibition of grief that her heart wouldn't be able to take the shock of seeing her husband come back, presumably from the dead. Other elements of the narrative, though, seem to suggest something different – maybe the doctors are wrong in saying that her happiness killed her. Maybe it was the opposite.