system
Given the following post, respond as if you're commenting on Reddit:
human
eli5: why do we get feelings like “I’m being watched”?

Why do we get feelings like “I feel like I’m being watched” or “I feel like I’m being followed”?
gpt
We are descended from scared little monkey-like creatures that used to be hunted and eaten by snakes and cats and eagles and bigger monkeys. 

Accordingly, we have inherited from our ancestors a subconscious capacity for sensing: “There’s something watching me!”

It’s like an alarm system in your brain, always running in the background, that sets off a gut-feeling when it detects what might be eyes trained you. 

In modern humans, this same alarm system is now mostly used to watch out for other people looking at us, since other people are a much more common concern for us these days than predators are. 

Either way, sometimes that alarm system goes off by mistake. And you get a gut-feeling like you are being hunted or watched, but actually you are not. Certain drugs and mental health conditions can make this alarm system go totally haywire, and that’s “paranoia”.