"No pain, no gain" makes for a great meme – pithy and catchy and rhyming and rhythmic – too bad it's also very inaccurate.
Gain comes from discomfort, not pain. There's a subtle but important difference between the two.
Pain – physical or mental – is a warning sign. We've exceeded our capacity, we're risking injury. When we feel pain, we need to stop what we're doing immediately, and figure out what's wrong.
Discomfort comes with all new experience. If the experience isn't new, like watching your favorite movie, it's comfortable. Comfortable can be enjoyable and restful, but all growth depends on new experience, like watching another movie by the same director. Growth – gain – depends on discomfort.
Discomfort and enjoyment can partially overlap – some people enjoy hard exercise, challenging music, strong flavors. But the same people might not enjoy deep stretches, the pressure of performing, meeting new people, etc.
The newer the experience, the greater the discomfort. It can be so intense it feels like pain. What's the difference?
Pain comes on fast, discomfort has a slow burn. Pain lingers, discomfort fades when the new experience stops. Pain is sharp, discomfort is dull.
Discomfort has the potential to get you something you don't have. Pain warns you you're about to lose something.
I don't have a good catchphrase to replace it though. Pain ain't gain? Discomfort for profit? I'll have to keep working on that one.