There’s a long pause. Dumbledore looks thoughtful. "Everyone here has a different reason," he says. "For example, two of my greatest lieutenants joined after they fell in love with a human. Lucius Malfoy, ruler of Hell, does not look kindly upon those who commit what he considers to be betrayals of the demonic creed by working actively for humans rather than against them. In fact, he calls us blood traitors. Others are in the Order because they believe that humans are a delightful and interesting race, and should be allowed to live their lives unmolested. Still others are... atoning, shall we say, for the sins they have committed in the past. There really is not a single answer, Hermione."