Humans have a natural curiosity - its a drive to learn patterns, rules, heuristic that lets you compress your understanding into something that's easier to remember. If you encounter a new rule that can better compress knowledge than the earlier one, that is 'compression progress'. Brain will use the better rule from then onward. We learn many such rules as children - but we refine them as we grow older. Brain rewards us with a good feeling or 'Aha moments' when we find a better rule.