The way that some people treat animals, dressing them up, doting over them, is (according to Lewis) a distortion of the first love: Affection. Affection is the love between a mother and a child; a professor and his student; the elderly man and his wife suffering from Alzheimers. It’s a wonderful love. It begins with the best intentions: e.g., to love animal found at the shelter. But like any other of the four loves, it’s spring loaded to turn in on itself. Each love (Affection, Friendship, Eros, and Charity) has its own unique way of doing this degenerative and ugly work. Affection, when not pruned and checked, turns itself into some form of, “Only I can give what you need.” See how this plays out, in his mind, with the way we treat animals,
If you need to be needed and if your family, very properly, decline to need you, a pet is the obvious substitute. You can keep it all its life in need of you. You can keep it permanently infantile, reduce it to permanent invalidism, cut it off from all genuine will-being, and compensate for this by creating needs for countless little indulgences which only you can grant. 
There is a much better way to treat animals, according to Lewis. This way honors “the beast,” and in a way, encourages us.
…the higher and domesticated animal is, so to speak, a “bridge” between us and the rest of nature. We all at times feel somewhat painfully our human isolation form the sub-human world — the atrophy of instinct which our intelligence entails, our excessive self-consciousness, the innumerable complexities of our situation, our inability to live in the present. If only we could shuttle it all off! We must not — and incidentally we can’t — become beasts. But we can be with a beast. It is personal enough to give the word with a real meaning; yet it remains very largely an unconscious little bundle of biological impulses.
It has three legs in nature’s world and one in ours.