> For that, you'd need to be convinced that exercise/strength training results in better health outcomes long term, and that better health outcomes are an important factor in emotional wellbeing.

On top of this, being physically fit generally results in increased longevity, giving you more total time to enjoy the things you do enjoy. (And enjoy them in a physically superior state.)

> Supposing that, it is safe to acknowledge that particular behaviour can be created by building reward circuits and overcoming initial aversion common when building any habit.

I think humans have a surprising capacity to learn to enjoy things (or, at least, tolerate) and that sticking with something is largely a question of overcoming an initial "enjoyment" curve. Besides, why does everything need to be enjoyable or stimulating? There is certainly value in a lack of stimulation.