"What does not kill me, makes me stronger"
Originally spoken by Friedrich Nietzsche. Although I do like the Joker's interpretation of it. And here's my favorite SK quote (not sure if this is correct, multiple versions of it on the internet):
"People want to know why I do this, why I write such gross stuff. I like to tell them I have the heart of a small boy... and I keep it in a jar on my desk."