Guys I just had a brainwave.
Everyone knows Batman's greatest villain, right? The Joker.
Well he knows the backstory of each of his enemies. Killer Croc, Bane, Hush, Black Mask, Poison Ivy etc. and uses that against them.
He knows that about all of them aside from one. The Joker.
in a similar aire to how nobody knows the identity of Batman, bar a select few, nobody knows the true identity of The Joker, aside from a possible few.
The reason that The Joker is such a foe to Batman, is that he recognises that a lack of identity allows him to be anyone. Just as Bruce Wayne went from a seeming nobody in the crime world, to the Caped Crusader, The Joker could be anyone.