>>43264307Nurse Joy got the egg from Sinnoh because it refused to hatch. Often trainers would come by interested in the egg but it would never hatch around them. Nurse Joy grew more and more concerned that the egg would never hatch.
Ash is currently in the Pokémon World Championships and had a battle with his next opponent. As we know from the movie and the DP special, Ash’s aura is unique and powerful, and the aura waves he gave off during the battle emanated with the egg, causing the egg to respond to it and eventually hatch into Riolu
At first, Riolu didn’t recognize Ash because his aura wasn’t immediately being given off, but when he and Pikachu came to rescue Riolu, the aura resonated again and Riolu agreed to fight with Ash, and then asking to join him.
As of right now Riolu is pretty weak so Ash only really picked up on its Aura when it was calling out to him while in the egg
