Most dogs are happy and they are "man's best friend" because they brighten our day. If you rescue a starving dog they will love you forever whereas if you rescue a starving human, in a week they will have decided they did you a favor. With cats, that sentiment lasts until the first meal is over.
But inside, your dog may be more pessimistic, according to a paper from Dr. Melissa Starling of the Faculty of Veterinary Science at the University of Sydney.
Dogs were taught to associate two different sounds (two octaves apart) with whether they would get the preferred reward of milk or instead get the same amount of water. Once the dogs have learnt the discrimination task, they are presented with 'ambiguous' tones.