SOOOO yea been watching a lot of batman lately and the line that the Joker says in The Dark Night "I don't want to kill you, what would I do without you?" made me think about the Jokers history and how no one knows who he is or where he came from and I came up with a "what if" scenario…here it is.
Before Bruce was born, Bruce's parents had a child before him, The Joker, and at a young age they gave him away for adoption and he was never re-homed or never went to a loving family which built up his "crazy" side.
As the Joker got older, he would visit his parents house and pier through the window. One day, The Joker goes to visit the Wayne's and see that they have another son, Bruce, and he see's how happy the Wayne's are with the new child, seeing that made his anger grow for the family which made him start to hold an almighty grudge against Bruce alone.
So that night when Bruce and his family went to the Opera, The Joker had hired a hit man to kill Bruce, but obviously the parents were killed instead, which made The Joker go all psychotic and crazy because he was the one that killed his parents, so he bottled up his past so that no one would know who he is or where he came from.
Just my theory tho..