system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 (or like... HS): Why does my stomach get upset when I'm stressed?

I just realized I totally take this for granted, but have no clue why it actually happens...
gpt
stress is a response left over from the era when it was very likely for a lion (or some guy who wants our food) to try to kill us with very little warning.  so when you feel like a lion could pop out at any moment, you become stressed.  when you are stressed, your body's long term functions (digestion, immune system, cell growth) start to shut down in favor of your short term functions (running fast, reaction time, breathing).  so when a lion finally did jump at you, you could run away faster then if you were trying to digest your food at the same time.  

however, that situation doesn't happen as much in america.  instead of lions, we have things like morgages and shitty jobs.  your body has the same stress reaction to these situations is it does to a lion, however, unlike a lion, these problems will remain for years at a time.  so your body is in full stress mode 24/7, ready for it's fight-or-flight response, and feels like that is more important then digesting you hamburger, which will just sit there for much longer then it should.