People who write Harry Potter fan fiction are all getting it wrong when they write up the effect Dementors have on people. About 93% of everyone that I read seem to think the Dementors feed on happy thoughts and memories and whatnot, and this is patently not true. Consider the Patronus, which is fueled by happiness and love; this drives Dementors away. If they really did feed on happiness, then the Patronus shouldn't drive them away, it should make them converge.
No, Dementors feed on misery, on sadness, on every negative emotion they can coax out of their victims. Their aura suppresses happy memories in order to force the negative ones to the fore; this is why people near Dementors start experiencing their worst memories. It's also a defense mechanism to preclude people from being able to pull up the happy emotions needed to fuel a Patronus.
I mean, really... how can people get it so wrong all the time ? It's common sense!