There's something about the puzzle. Looking at my old, deranged coding workarounds that tried to solve problems that have already been solved a thousand times before with established paradigms; and then moving them toward said best practices; and doing it in a way that no NEW technical debt is created. It's just satisfying.
I think one of the best learning experiences for me has been the fact that I created a bunch of sloppy shit by hand, auth and all, which forced me to learn things the hard way. All along the way, people were shouting from the rooftops: "use established libraries, dummy!", which is the same advice I would give to somebody today. But by doing things the hard way, I learned so much more about the inner workings. And, I've also given myself a decade's worth of refactoring work, which I really enjoy!