The healthy body we maintain costs a lot in terms of energy and nutrients. Thus it makes sense that our bodies would constantly look to shed anything it doesn't need to survive. In the past that meant life or death. So in essence the body is continually stripping what is not essential. It judges what's essential in reference to what we do. So we can influence it.
Thus if you want a strong body that's fit for daily challenges you need to put your body through some challenges to encourage it to make itself strong.