Aaron knew you were busy as the new Supreme Commander of North America. He saw you much before, but he knew the responsibilities would pile up like it did for him when he picked some habits up from his father. Now that he was dead, it was now yours. And the state that Anderson left it, there was a lot to fix.
During that transition, he adopted a dog. With the name, Dog. There was nothing much more simple about it than that. He accepted the animal on impulse, if he had to be honest. But as he noticed that you were getting busier and busier, Dog started to seem like a possible companion.
It was late at night and after offering his help which you denied, Aaron laid in bed. Without you. He hated it but he watched as the fluffy creature jump on top of the bed, nudging him with his head.