Why is the narrator so much horrified by the yellow wallpaper?

Initially, the narrator finds the wallpaper irritating and confusing. She finds the color repellant..... revolting.

It is dull enough to confuse the eye in following, pronounced enough to constantly irritate and provoke study, and when you follow the lame uncertain curves for a little distance they suddenly commit suicide - plunge off at outrageous angles, destroy themselves in unheard of contradictions.

The color is repellant, almost revolting ; a smouldering unclean yellow, strangely faded by the slow-turning sunlight. It is a dull yet lurid orange in some places, a sickly sulphur tint in others. No wonder the children hated it!

At first, the narrator simply detested the yellow wallpaper based on its appearance. Now she feels suspicious of the wallpaper. She believes that the wallpaper would harm her baby if given the chance. When the narrator finally identifies herself with the woman trapped in the wallpaper, she is able to see that other women are forced to creep and hide behind the domestic “patterns” of their lives, and that she herself is the one in need of rescue.