See the thing is, Harry, there’s some wizards, like the Malfoy family, who think they’re better than everyone else because they’re what people call “pure blood. That’s horrible! And it’s codswallop to boot. "Dirty blood.” Why, there isn’t a wizard alive today that’s not half-blood or less.
Dementors are among the foulest creatures that walk this earth. They infest the darkest, filthiest places, they glory in decay and despair, they drain peace, hope, and happiness out of the air around them … Get too near a Dementor and every good feeling, every happy memory will be sucked out of you.