system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Why is slouching so comfortable when it's actually bad for you?
gpt
Slouching is comfortable for some people because it takes less effort and (short term) your bones are still within a normal range of position.

You're basically trading off some muscular stress for extra bone, tendon, cartilage, and possibly nerve stress.  These parts: muscle, bone, cartilage, tendons, and nerves, have different tolerances, responses to stress, and wear differently.  Some of these parts can suffer (sometimes severe) damage for prolonged (even light) stress.  

Muscle is more adaptable than the other parts.  So, it's best to stress muscle most of the time.  You get the side benefit of getting stronger and looking more attractive.