"One of the things that's been nibbling at me, though, is this very thing - why is Krona doing this, when at the end of that story, he got what he wanted?"
There come for some a point when the journey itself becomes more important than where one is journeying to. Therefore, once the goal is reached, you think "now what", reject what you have, and rather than find a new goal act like you haven't found it yet and go back to looking.