The physical characteristics of dogs are pleasant, they are usually soft to touch, warm, and never answer back. They are a stimulus for exercise and keep their owners active. They give protection and impart a feeling of security. They can improve self-esteem, and may have the ability to reduce stress. All these features can be provided at rimes by another person, by a dog constantly.
Humans are able to form a strong emotional bond with a dog, and the dog with the human. That perhaps is the key to the whole relationship and why it is so successful. Rudyard Kipling said it in another way in The Power of the Dog: 'Brothers and Sisters, I bid you beware of giving your heart to a dog to tear'.