John does not know how much I really suffer. He knows there is no reason to suffer, and that satisfies him.
John, the narrator's more logical counterpart, often tells her there is no "reason" to suffer. She has a nice home, food to eat, clothes to wear and a husband who "cares about her very much" - so what reason does she have to complain?
Men, exemplified in her husband John, had a hard time believing that women could want anything outside of a trivial domestic existence - that that should not be satisfying enough. Therefore, the rest cure seemed to be an effective treatment. What detriment could denying women any exercise of the mind have, if their biology dictated that they should be satisfied by domesticity? A woman doesn't have to read, write or work to be happy - in fact, it is probably the cause of her unhappiness.
How men and women suffering from depression were treated shows how differently they were viewed by the medical community. While women were prescribed the rest cure, men were perscribed the "west cure" - they were instructed to travel west, engage in physical activity and then write about it. Men were told to be more adventurous and engaged. Women were told their depression stemmed from being "over-stimulated". They, like our narrator, were instructed to disengage from the world.