John, the narrator's husband, is a doctor and so has high status in the culture of the time. His diagnoses are considered binding, since there is no possibility that his wife could understand her own mental state, and so he confines her to a mansion as a method of curing her "hysteria."
He says that with my imaginative power and habit of story-making, a nervous weakness like mine is sure to lead to all manner of excited fancies, and that I ought to use my will and good sense to check the tendency. So I try.
I think sometimes that if I were only well enough to write a little it would relieve the press of ideas and rest me.
(Gilman, "The Yellow Wallpaper," library.csi.cuny.edu)
Because the narrator is so mentally creative, she is overwhelmed with constant mental stimulation even in a confined area; this is the direct cause of her eventual breakdown: she fixates on the one unusual thing in the house, the wallpaper. However, John takes no notice of either her well-being -- taking it for granted that her isolation is helping her mentally -- or her fixations. He believes that the act of writing is bad for her, instead of acting as a mental outlet for her ideas. John is so bound by his cultural blinders that he cannot comprehend her needs. By isolating her as a child instead of speaking with her as an adult, he increases the mental pressure and speeds her deterioration.