Not forgetting "Out of life's school of war: What does not destroy me, makes me stronger. " - Nietzsche, Twilight of the Idols. Evolution is a full & succinct answer, I'd say.

@CriglCragl - How could evolution and genetics possibly facilitate this rule?

I think this question is another in a line of “how does science explain this completely-nonscientific concept?”. I have an answer that clarifies why I think that you’re going a bit too far by calling it a “universal rule” but the fundamental answer that this question and others like it will get is the same: physicists doesn’t explain non-physics.

To evolve resilience, requires evolutionary challenges. Very stable adaptive niches generally produce species that are so well adapted they struggle with change.

For the record: your last few questions have all been in that line of “how do physicalists/does science explain X?” for a non-scientific concept X, and they’ve all been downvoted and closed. I would recommend taking that as “these kinds of questions aren’t on topic for the community.”

I’m voting to close this question because the question is based on false premises and therefore has no answer.

@controlgroup Have you ever done any exercise other than physics?

I also don't see why a physicalist answer to this would differ from a non-physicalist answer. It not only isn't science, it's barely philosophy. It is an overly pithy saying that means nothing more than "improving takes effort, which can become uncomfortable, especially if you don't know when to stop and rest." Truism, not Truth, and not even always lowercase-t true.

If the question is, "why is it that there are some gains whose getting is physically painful or mentally unpleasant?" then the physicalist answer would be to propose that the answer lies in evolutionary biology and evolutionary psychology respectively, and direct you to the relevant stacks. Unfortunately ev. psych. is more like a collection of naturalistic myths than a science, but it's the best we've got at the moment if you prefer white coats to white collars on your anointed keepers of lore.

I happen to quite like the just-so story the ev.psych people would tell you: Those gainful activities whose nearest analogs would not have produced gains if done in the ancestral environment are mentally unpleasant because doing them reduced procreation outcomes and thus they were selected against. In the distant future we'll all hate condoms and get a huge kick out of eating spinach for breakfast for much the same reason as we now hate getting up before dawn to go sit in a box all day, and get a huge kick out of frosted sugar bombs. And that's how Tiger got his stripes, just so.

The biology answer (re physical pain, especially in exercise) is worth getting from Biology SE.

@gs would I get from biology SE....for example.. how a rose gets it form and colour? like a tiger gets it form of stripes and the colour of its eyes and our valence toward them whether they're pleasing to the eye or a warning of pain?

@8Mad0Manc8 Roses are a domesticated plant that many generations of gardeners have selected for their forms and colors, so the most salient How would probably be found on History SE, not Biology, although Biology SE could probably tell you some interesting things about them too. (My tiger comment was a joke about satisfying but baseless stories about why the world is the way it is.)

@gs so a domesticated or wild flowers valence can only explained by a biologist?

Principle of conservation of quantity of pleasue, where pain=negative pleasure.

@JD since you added corroborating refs here is one more direct one: Pain is the remedy, and pleasure the disease for in pleasure God is forgotten — Sikh text Asa Di Var.

@Rushi perhaps it is more like: with satisfaction of wants God is made 'redundant'? But I suppose some pleasures are extreme and destructive: with too much opioid, pretty much everything is forgotten, sometimes permanently.