Yes, this. Cats can be stabilizing too. I can't be depressed around a cat. I haven't had a manic episode for several years so I don't know if cats have anti-manic properties or if I've just been lucky. But pets are great for mental health, no question.
His experience is different from mine. For him, the dog is a neutral character (a generally innocent animal of fairly average temperament) he has intense emotions toward, and he's begun to recognize that negative feelings toward the dog indicate a deteriorating mental state. For me, I rarely feel negative emotion for any animal except humans, and cats (and, to a lesser extent, dogs) are a stabilizer, a reminder that happiness (and not that fleeting manic kind) exists in the world. I'm pretty sure that one of my cats is a Zen master (except when in the presence of boiled or fried chicken).