When I was young, and first saw the Joker, I freaked. He was the scariest character of my childhood.
When I was young, I read the adventures of Batman, hoping that he will one day kill the Joker. He never did. The Joker shot Barbra Gordon through her spine, and killed one Robin. He also performed plastic surgery on another Robin so the boy could look more like Joker than Batman. Batman stuck to his "no kill" code and let Joker live on.
Now I see the mythology (it's probably a little early to call Batman a mythology, but I'm sure this character will last as long as Romeo and Juliet, or Hercules) as something else. Now, I know that Batman let Joker live not because he wouldn't kill the clown, but because he couldn't.
This is my interpretation of the mythology: Gotham, the city that housed Joker and Batman, represents our individual bodies. Inside our Gotham, two characters battle for our cities. Inside each of us, live a Batman and a Joker. In a way, it's like some religious Good vs Evil, except that heaven and hell exist while we're alive. If your Joker wins, then it's hell on Earth for you. If Batman fights, then your Gotham is heaven on Earth.
Everyday, our Jokers will tell us to give up, to give in, because the only truth is Chaos, because all living things are inherently selfish, and we are born into a meaningless existence of which Death is the only certainty. Our Jokers constantly tells us that we're never going to measure up, that we're not good enough, and therefore we might as well go nuts. If one succumbs and gives in to his Joker, his Gotham becomes Joker's playground. You can see evidence on the streets sometimes: some madman walks by muttering to himself or herself. That's a Gotham that's completely, irreversibly overrun by its Joker. For most of us, it's not that extreme - for most of us, our Jokers only managed to stop us in achieving our goals and dreams. "I can't live a life I want, because ______________ (fill in blanks with a reason. Any reason will do!)." or "Things will never change, because _______________ (again, any excus . . . I mean, "reason", will do), so why bother?"
Or we can listen to our Batmans. We can believe that our Gotham is a good one. Our Gotham can succeed. Our Gotham is the city where our lives are good and our dreams come true.
It's going to be a constant battle without end (well, it ends when we die). Our Jokers may never die. No matter how strong our Batmans are, our Joker will always be looking for opportunities. Everytime we feel defeated, we can hear a voice whisper, "See? you cannot win. Don't even try!" When someone else encourages or helps a man in ways that counters his Joker's intention, his Joker may even retaliate and do vicious harm to the person.
Few of us can really get rid of our Jokers - even great men and women wrestle with self-doubt and question the point of their struggles - but many of our Batmans managed relatively well. How much each of us can achieve our goals depend largely on our Batmans' ability to keep the Jokers at bay. If our Batmans are doing great jobs, we may actually shine so brightly, we touch the lives of those around us.
I believe in our Dark Knights - civilisation did not come such a long way when Jokers are trumping overall. We'd trumped the Nazis, the racists, and sexists. But there is always room for improvement.