I'm convinced that there are no sane people in the world. Everyone's a lunatic. Some people are just better at hiding their madness.
If you get to know someone well enough, once they share with you their secrets, the weirdness of their minds, and you start noticing their peculiar idiosyncrasies that make them the person they are, you realize that they are as abnormal as you are, and you get comfortable sharing your insanity with them.
That's probably what defines a friendship: knowing each other's crazy and loving it.