For the human body not to have acute, intermittent stress is to cause atrophy of muscles and a weakening of the mental state–(you do need time to recover between stressful events). You grow fat and lazy (and unhealthy!) in the absence of good stressors.
There are, however, bad stressors. Continual and nagging stress like self-guilt, a difficult boss, continual bills/debt, etc… are all bad because they give you no reprieve. Rest is paramount.
The more weight you lift, the more your bones and muscles anticipate will need to be lifted next time so they grow stronger.
Your mind also mentally fits itself to take on more difficult tasks the more you take on difficult tasks.
Take the Indian woman who has been carrying a 5-gallon jug of water on her head since she was a young girl. She’s 75 years old now and can still hold 50lbs up there. She has great muscle and bone strength and she lives in a continuous state of on-again, off-again physical and mental stress with periods of rest.
Compare that to the typical 75-year-old American who is two decades into retirement and living in a state of perpetual rest. He is growing weaker every day. His muscles getting flabby and his skeletal structure has grown weaker (not stronger!) He no longer has the will to challenge himself and his mind is losing cognition. He lives in the absence of stressors.
The key here is to have stressors (physical and mental) with periods of rest and recuperation in between. Weight lifters make really heavy lifts and then spend the rest of the time resting. Rest!
I’m not a scientist, but the optics of the absence of stressors and the weakening of the person (or society) that craves tranquility all the time is an interesting juxtaposition.