Joe: Rule number one--a Watcher and an Immortal can't be friends.
Duncan: Come on, Joe, you don't have to choose.
Joe: Don't I? I took an oath and I broke it. The Watchers have been around for a long time. What gives me the right to say, "We'll do this my way"?
Duncan: Because you're a man, and part of life is deciding, making your own judgment calls. Like choosing who your friends are.