This picture tells a thousand words. The two of them must have gone through a lot in life to form such a deep alliance. Her subtle way of touching his formidable whiskers is a way of letting him know how much she cares about him. The dog meanwhile remains gentle with his small friend, knowing full well that he can easily end its frail life with the swipe of his paw. But he chooses not to.
Because he is tame. Because he is civil and docile, unlike the savage street dogs he has seen hunting for prey and small creatures. He fears ever having to go back to that savage lifestyle, loving this cat is his way of driving away that instinctive fear. We hope the dog never has to go back to his bitter, savage way of life.