Coming clean
Don explained that he realized that Cammy didn’t like his prosthetic arm and thought that there was something wrong with him. So each time he saw Don he would freak out and bark at his arm. Don didn’t want to talk about his arm that much in a serious manner, it was a painful reminder to him.
The most he had ever done is a joke about it, but in all seriousness, Don was traumatized after losing his arm. He found out that when he took it off, it would calm Cammy down and he would act like a regular dog towards him. In some way, it made Don feel like a monster.