Albus Dumbledore

Dementors are vicious creatures. They'll 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 is not in the nature of a dementor to be forgiving. But you know, happiness can be found... even in the darkest of times... if one only remembers to turn on the light.