>>53056358He was forced by his dad to take care of it when he was still a little kid. To him, it was basically the embodiment for his dad's obsession with his work and thus the reason he neglected Arven. It's honestly understandable that he hated it.
But he forgives Miraidon later in the game and says it was a mistake to blame it for his dad's absence. He just needed a scapegoat.
Also this is something that I realised much later, but the reason why the Raidon likes sandwiches so much is probably not because of us. It's likely that Arven fed it sandwiches when he had to take care of it, and maybe that's why it likes them so much.
