Long-term psychological depression in dogs is very unusual but not unheard of. The most common cause of depression in dogs is the loss of a close human or animal companion.
Sudden and significant changes in a dog's life, such as moving to a new home, can cause depression. A drastic change in routine, such as an owner starting a new job with a different schedule, can also bring on a period of depression, as can the addition of a person, such as a new spouse or baby.
Symptoms of depression in dogs are the same as in humans, and they can include listlessness, social withdrawal, refusing food or water, a lack of interest in favorite activities, sleep disturbances and inactivity.
If the dog has become depressed because of the loss of a fellow pet, adopting another dog can provide distractions, provide companionship and improve the depressed dog's mood. Extra play time, exercise and attention can help dogs recover from depression. A person should reward a depressed dog with treats when he begins to show signs of happiness and interest in activities in order to reinforce those behaviors. Since the symptoms of depression can mimic many physical illnesses, pet owners should consult a veterinarian to confirm the dog's condition.