Part of the Plan.

Often, the Joker would overlook the city of Gotham at night, thinking, contemplating, planning. For a while, the Joker was just a dog chasing cars when he was involved in the mob. Eventually, he met a catalyst-- a woman. This woman seemed to know everything he thought, the way his mind ticked. She put him on edge and caused him to ponder things he never had before. But, the question that constantly plagued his mind was not "Who is she?" "What's her significance?" or even "Why a woman?" but instead "Is she real?"