system
Given the following post, respond as if you're commenting on Reddit:
human
[Harry Potter] What exactly are the dementors? Do they work for the government as prison guards or are they feral beasts who just happen to live near a prison as an escape deterrent?
gpt
They appear to be a sentient and very much malevolent species. Dementors drain happiness from their victims to sustain themselves. That happiness needs to come from somewhere. So, in an attempt to prevent them from terrorizing the countryside the ministry has them guard Azkaban where they can drain all the happiness of the prisoners nonstop. Which is horrifyingly inhumane if you think about how Azkaban seems to be the only wizard prison in Britain.

They allied with Voldemort (both times) because he allowed them to run wild while the ministry kept them on a comparatively short leash.