Monomania

the state of a person being obsessed with achieving a single desire or goal, usually the face would be willing to do anything in order to fulfill their need

person 1: gosh, there's this car i've wanted to purchase for years, i've worked my ass off every day just to get all the money. honestly, i'd even sell my organs for it.
person 2: jeez, dude, that's a real monomania if you ask me.