Dumbledore: A word of caution: dementors are vicious creatures. They will not distinguish between the one they hunt and the one who gets in their way. Therefore I must warn each and every one of you to give them no reason to harm you. It's not in the nature of a dementor to be forgiving. But you know happiness can be found even in the darkest of times, when one only remembers to turn on the light.
Harry Potter and the Prisoner of Azkaban quotes