If you live with a dog you just know when it's happy or miserable, don't you? Of course you do.
People have had a close bond with domesticated dogs for centuries.
In his 1764 Dictionnaire philosophique, Voltaire observed: "It seems that nature has given the dog to man for his defence and for his pleasure. Of all the animals it is the most faithful: it is the best friend man can have."
Research has shown time and time again the positive impact pet ownership can have on our lives.
Indeed, a study of 975 dog-owning adults, found that in times of emotional distress most people were more likely to turn to their dogs than their mothers, fathers, siblings, best friends, or children.
It's not surprising then that dogs are now the most commonly used animal in therapy.
Our canine pals are being increasingly used as participants in a variety of mental health programmes – offering companionship, happy associations and unconditional love.
In the UK, Pets As Therapy (PAT) has more than 5,000 active PAT dogs, which meet some 130,000 people a week.
In the US, the American Kennel Club has a Therapy Dog Program which recognises six national therapy dog organisations and awards official titles to dogs who have worked to improve the lives of the people they have visited.
Dogs who heal
Sigmund Freud is generally acknowledged as the accidental pioneer of canine-assisted therapy.
During his psychotherapy sessions in the 1930s, a chow chow called Jofi stayed alongside him in the office. Freud noticed that patients became more relaxed and open when Jofi was present, and it helped him to build a rapport.
But the official beginning of animal-assisted therapy is generally linked to World War II, when a Yorkshire terrier called Smoky accompanied corporal William Lynne when visiting service hospitals in New Guinea. Her presence lifted the spirits of wounded soldiers.
Despite all this, it was not until the 1960s that the first documented case study of a dog working as a "co-therapist" was made.
The US psychotherapist Boris M. Levinson maintained that the presence of his dog Jingles added a "new dimension to child psychotherapy". Despite opposition from peers, Levinson strongly defended the use of dogs as therapeutic aids.
How dogs feel
But while there is no question that dogs are very good at understanding us, sadly the reverse is not always so true.
A classic example of this is when someone has had a little "accident" in the house and dog owners think that their pet looks guilty.
But for the dog in question, that look is purely submission and is a way for the dog to say "don't hurt me" rather than an admission of guilt.
It is very difficult for humans to convince themselves that the canine brain is not able to understand the concepts of right and wrong – but without that ability it is not possible to experience guilt.
The dog who is looking guilty is simply afraid of your reaction to the situation – usually based on past experience.
Some of the main difficulties that happen between dogs and their owners are caused by a humans inability to read their pet's body language correctly.
Combine this with the human notion that dogs understand abstract concepts and can use reason on complex issues, and the scene is set for problems.
Among other functions, this hormone is thought to help relaxation. It helps to form bonds between mother and child – and between pet and owner.
So although we can't know for sure how a dog feels during pleasurable activities, it seems reasonable that oxytocin produces similar sensations in dogs to those that humans experience – suggesting that they are feeling affection towards and attachment to their owners.
Similarly, dogs that are in unpleasant circumstances show raised levels of the stress hormone, cortisol. One of the situations that produces this stress response is being left alone for any length of time.
Dogs are pack animals and really need to have company. A solitary dog is rarely a happy dog – and this is something that all dog owners should take into account when planning their lives.
What this all shows is that for dogs and people to live together and work together – and for both parties to be happy about it – an understanding of each other's emotional state is vital.
Even if dogs and people don't completely understand each other, it seems clear that each species is essential to the other's well-being and we can help each other to be happier and healthier.