I think the most interesting thing about Hoarders is how hard it is for the people to separate from their things. Even the smallest piece of trash has some kind of meaning, and they argue within them selves to throw it away or keep it. Sometimes they keep the most ridiculous items, but to them its a memory of something minor, or they throw the item away and move on and don't seem to dwell.
Letting things get so out on control is just because they can't seem to part with the trash, and they it just accumulates to something worse. I remember hearing stories, of how family members would clean their the place, then when the Hoarder came home from work and saw they clean house they lost it!