"I try. See, the thing is - I don't like it when things I want don't go my way. It's not, uh - it's not good for me, and I look out for my self-interests. Y'know how it is."

The thing about Crane, Joker figured to himself as he mimicked the doctor's usual method of spraying noxious fumes into the other's face over their breakfast, was that he took the Joker's intelligence for granted. Assumed things, like maybe he wasn't going to eventually use the laughing gas he had given him against the doctor himself at some point - or that he would be too stupid to figure out how to do it. Either or.