system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does being sedentary degrade our bodies and deteriorate our health faster than being physically active would if exercising is more directly stressful on our bodies?

Now that I think about it, it's seems counterintuitive that causing your body to work more will preserve it better than being less physically in the long run (assuming that you have the same body mass and composition in each case). Why is that?
gpt
In essence, it's because your body is lazy, and it's evolved to be. In the environment we evolved under, being sedentary for long periods of time was simply not something that was possible (you'd run out of food and die) - so which muscles and other tissues you regularily used was a pretty good measure for which ones you needed. As a result, muscles you aren't using or are using less essentially gets maintained less well to save energy, because your body doesn't think you need them. Evolutionarily speaking, being efficient and saving energy is far more important than maintaining less-frequently used tissues.

On a more mechanical level, a lot of the mechanisms that repair and maintain your body rely on signals that are produced by exercise to get started. (This is also why physical therapy is important for healing many injuries - without the stresses of appropriate exercise a lot of repair mechanisms simply don't start.) A lot of cells in your body can sense mechanical stress and respond to it, and quite a few growth and repair pathways are the result of such responses.