The Hatred Between Man and Monster
In Monsters, Inc., it’s pretty clear that humans and monsters don’t like each other. However, monsters found a way to benefit from their fear of humans—by turning their screams (and laughter, but we’re getting ahead of ourselves) into energy. Now, that might seem to be the extent of the monster-human relationship, but in the DVD, it actually goes beyond that.
Apparently, monsters and humans used to live side-by-side in total harmony. But for some reason, humans would turn on monsters out of fright, banishing them to a fictional universe, which is only accessible through interconnected doors. We’re not sure how this helps the film’s plot, but it’s a pretty neat nugget of knowledge.