noun
that which preoccupies one’s mind or emotions excessively or abnormally.
She no longer cared about money and success in and of themselves; surpassing her sister in every way was now her obsession.
His friend’s wife had become an obsession, and try as he might, he could not put covetous thoughts of her out of his mind.
My roommate’s obsession with neatness is driving me crazy; I can’t put a cup down on the table for a second without her putting it in the dishwasher.