Why does the narrator become obsessed with the wallpaper?

The narrator is suffering from post-natal depression and has been prescribed a 'rest cure' by her physician and her husband. She is a woman of creativity and great imagination who finds this isolation suffocating. Desparate for a focus, the 'hideous' wallpaper captivates her imagination-

'I never saw a worse paper in my life.

One of those sprawling flamboyant patterns committing every artistic sin.

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 repellent, 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! I should hate it myself if I had to live in this room long.'

Unfortunately she is confined for a long period, and her obsession with the paper, in the absence of any other diversion or stimulation, becomes deeper.