my theory was about Ash being depressed because he was destroyed by Tobias. like it was embarrassing. so Ash had started to think that he isn't destined to be a great Trainer so he kind of just traveled through Unova with a focus on having fun and basically taking a permanent vacation from training. but then he realized (sometime between the Unova league and Decolore) that he is still young and has a lot of potential, which is when XY starts. but there weren't any signs that led to the depression. like Ash was still happy as always and really had zero personality changes...