Joker thinks that there's more evil in people than good. He brings out the evil in people just like how he made Harvey dent go mad,just like how he wanted batman to kill someone which is against his morals but ultimately making him kill Harvey dent he brought out the evil in him. When Bruce's parents were killed in was on the verge of becoming mad and evil against the society which killed it's parents. Joker thought he was successful in bringing out the evil in him. But Bruce Wayne has grown beyond the evil and become the superhero for Gotham. This is what makes Bruce Wayne character stronger than joker.