Much as people who don’t go to the gym like to think of working out as mindless pumping of iron (which it is for some) the reality is that, as with everything, once you progress beyond being a beginner, there is much more to the activity than perhaps meets the eye.
It’s the same feeling I get from combat sports; whilst I’m at the gym, I can concentrate on something: form, speed, power, strength…
Being strong, for example, isn’t just about gritting your teeth and contracting your muscles as hard as you can. What most people don’t realise is that pure strength isn’t just muscular – it’s neural. Strength is about coordinating as many muscle fibres as possible at the same time, which relies entirely on your central nervous system (CNS).
Sure, if you’ve trained those fibres, they will contract more forcefully and the movement will be stronger. Yet ten fibres contracting at the same time still creates more force than three fibres contracting thrice as forcefully. This coordination comes from practise, but it also comes from concentration.
To this extent, lifting – especially heavy lifting – is a mental thing. That isn’t to say working out is like doing maths or philosophy – it’s nothing like that at all. What it does require, however, is intense periods of concentration and effort – which leaves no room whatsoever for musing on the meaninglessness of daily life, obsessing over little problems and getting wound up over things that, in reality, shouldn’t bother you at all.
To that extent, I find working out, weight lifting and exercising in general hugely rewarding. Perhaps you do/will too.