he explained at harveys bedside that he liked fucking up peoples plans, that was always HIS plan. to show people when things dont go as they want/foresee, something as dumb as spilling coffee on yourself ruins an entire day.

also- in the beginning when joker walked in on the mob meeting (chinese dood on the tv), he said basically he did jobs for people. he gets paid but doesnt care about using the money, its when money gives people the idea of power, THATS who he likes fucking with. this showed again when the chinese guy was sitting at the stop of the bill pyramid, and greaser mob guy got all salty when joker started burning the money.