Well, Joker kinda has a reason: his dad was an alkoholic, his mom a drug-addict which damaged his psyche. Maybe he does it to show he's not a nobody that is afraid and destroyed?
The joker does it for self actualization he believes that everyone including him us inherently broken and he tries to bring this out of them. He just wants everyone to know that he's not crazy that he's what we all hide inside ourselves
:)