Your dog can never lie to you. It is physically impossible. Think about that for a moment. If you treat it properly, your dog will spend its entire life loving you unconditionally. And never tell you a lie.
It will let you know if it’s sad, or if it’s sick, or if it wants to play, or if it just wants to curl up in your lap. Complete honesty. No deception. Do you have any human relationships like that? Doubtful. Even your spouse may keep a few things from you, or tell you a little white lie on occasion. As you likely do to him/her.
Dogs can be completely honest because they are just happy and grateful to have the simplest things in life: shelter, food, and someone to love. We could learn a lot from our dogs.