Filch is a Squib!
I've always been puzzled by the idea of Dementors 'feeding' on happy memories in the first place. If that was true, how could Patronuses, conjured of happy memories, drive them off?
Originally Posted by Tim the Enchanter
I've always thought it more likely Dementors feed on the despair people, drawing out their worst memories and forcing people to relive them until they simply can't remember what it's like to be happy. I know that's not quite how the process is described in the books, but the effect is the same, and I think it might just be two different ways of explaining the same thing.
If that's the case, then an excess of 'despair and degradation' would be beneficial to the Dementors, and by simply hanging around they would increase their own 'food' source.