Dialga had seen countless trainers journey through his land of Sinnoh. Observing these kids of careless parents provided him with unmatchable entertainment. He loved making these kids' lives hell. As a result, they found themselves at all the right places at all the wrong times. Their mindless scramble to look for Gym leaders that were away or search for another path where the obvious one was conveniently blocked brought the sadist immeasurable joy. In the land of Sinnoh, Life did not happen to people when they were busy making other plans. Dialga did. He loved wasting people's time, especially those who valued it. Then he met Ash Ketchum.
At first he looked ordinary enough: A trainer from Kanto starting a new journey. He met a few friends on the way and they all went on an adventure. Then Dialga started playing his little game. Only problem was, it backfired.
Ash was unlike most Pokemon trainers. He was in no hurry to get anywhere. No matter what distraction Dialga threw at him, Ash had his fun time with it. Throughout Ash's journey, Dialga kept on trying to annoy him but Ash simply refused to give up. So he went to look for his friend Palkia and ask him for help. Together the duo put this little child through unimaginable torture of time and space, putting him at all the wrong places at all the wrong times. He kept on running into stray pokemon, upset kids and old people who needed his help in some way or other. It was seldom that he found a Gym Leader he could challenge before doing some kind of errand.
But this child was made of some strong metal. Inspite of the 3 months it generally took him to reach a new town, Ash finished his adventure in Sinnoh and was ready to depart for elsewhere. However, unwilling to accept their defeat, the two Deities in their rage laid a curse on Ash. They reversed Ash's internal clock, making his age go BACKWARDS until he became a little sperm and finally ceased to exist.
And that, my friends, is the only logical explanation as to why Ash is 10 again in Best Wishes.