system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If bad posture is so bad, why does it feel so good?!
gpt
Because your body is used to it. if you slouch all the time and then try and sit up straight, you're forcing seldom used muscles to work. If you sit up straight all the time, slouching will hurt because your back muscles will stretch awkwardly in an unfamiliar way. If you sit for a long period of time, you'll probably be a little sore regardles, but more so if you're slouching, which puts pressure on your spinal nerves.  

when you'd sit straight all the time and have the muscles to support yourself,  you realize that slouching actually doesn't feel that good.

tldr slouching feels good because you haven't experienced truly comfortable sitting, maaaaaaan.