There are few relationships between fictional comic book characters that have been more analyzed than the one between The Joker and Batman. On the surface, the two characters appear to be polar opposites, with Batman being a rational, ethical thinker who lives in order to preserve order, and The Joker being a complete psychopath who exists to create chaos. However, upon further examination, it appears that the two are not necessarily polar opposites, but are actually mirror images of one another, and this is likely why they have had such an interesting relationship throughout all of the stories that have been told about them for more than seventy years.
While the story of how The Joker became The Joker is largely unknown, it is apparent in a number of stories about him that some sort of tragedy in his life inspired him to become who he is. Batman, of course, was also inspired by tragedy, but he took a completely opposite approach to it than The Joker. Either character could be said to have suffered some sort of severe crisis in his younger years, and then had to struggle with their own view of the world in order to come to their own conclusions. While Batman struggled within his own mind to hold on to his traditional ideas of right and wrong, The Joker threw those ideas out the window and embraced not only philosophical nihilism, but also active nihilism.
The conflicts between Batman and The Joker speak to our own struggles when it comes to dealing with tragedy and trying to maintain a moral code. The Joker maintains that anyone could possibly go through similar things as he did and become like him, while Batman embodies a character who has overcome his personal tragedy and has been able to keep his own values intact.