This is pretty much the reason why people quit gym before seeing success, as those who have been going long enough to get reward from it, know they have to keep it up, else lose those rewards. For those who haven't seen success, it starts off as a "think I can" and can all too quickly fall into a "I can't" if there is no reinforcement (I've always maintained that if you could give someone results straight away, they would never quit...hence I stress focus on strength goals to distract from the slow onset of physical changes).
Yet even for those who have seen success, it's easy to fall into the trap of believing in your limitations, and so your accountability towards your goals takes a dive.
Same goes for coaching; there's more accountability in it when you are lifting not just for yourself, but to meet the expectations of another. This is why I've seen more improvement than ever, since documenting my lifting on here: I have to lift the weight or people see that I'm all talk hahaha
As for people who do incredible things, they are usually the ones who won't stop until they are satisfied, and for them, satisfaction is a dangling carrot that will never be caught, but must forever be chased after. You may never catch it, but you will end up miles away from mediocre.