system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why we feel like there's a monster behind us when going upstairs in the dark.

Because I'm fuckin' 20, bro and that shit still gets me. wtf bro.
gpt
[Speculation alert]

There's probably an evolutionary benefit to being wary of having your back to an open area of darkness, since in the ancestral environment there was a non-trivial chance that in a 'going upstairs in the dark'-type situation, there actually is a 'monster' (predator of some kind) waiting to strike. Having the feeling of something behind you means you avoid that kind of vulnerable situation more, you move more quickly, and you are more alert and ready to respond if there turns out to actually be a predator there.

Evolution is not tailored for your comfort, but for your survival. It's better to be nervous for no reason, than to get eaten because you felt you were safe.