system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do we lose our appetite when we become extremely upset?
gpt
It's adaptive. When an animal is in a stressful situation (danger which might require a "fight or flight" response) it makes sense to de-prioritize eating temporarily. When people override this natural inclination repeatedly (or do not experience it to begin with due to biological abnormalities), that can result in an eating disorder (stress eating) where stress actually triggers and increases hunger rather than the reverse, which often leads to dangerous levels of obesity. Particularly because the physical and emotional stress resulting from the obesity can then trigger hunger, an especially treacherous feedback loop.

If you are looking for scientific information on the hormones and other neurotransmitters which implement this mechanism, you should try Google.