How we cite our quotes: All quotations are from Harry Potter and the Prisoner of Azkaban.
Quote #4
HARRY: But someone was screaming. A woman.
For Harry, the Dementors are even more terrifying, because he hears the screaming of a woman whom he believes to be his dead mother. Yikes. That's the stuff of nightmares.
Quote #5
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.
The Dementors make Hogwarts into a terrifying place, instead of a safe haven. (We still think that a Dementor-infested Hogwarts is less horrific than, say, an abandoned amusement park.)
Quote #6
DUMBLEDORE: But you know, happiness can be found even in the darkest of times if one only remembers to turn on the light.
Our resident Yoda is basically saying that, even when you're scared, you need to look for something positive to get yourself through the darkness. Easy for one of the most powerful wizards on the planet to say!