While your dog can't tell you when he's feeling down in the dumps, he can feel sadness just as easily as you can, especially in the wake of change. Dogs thrive on habit and on the companionship of their loved ones, be they humans or other pets. Major life changes can sink them into a depressive state.
Causes of Depression
Dogs can go through periods of depression, particularly when they have to deal with change -- particularly major life changes, like moving, a baby joining the household, or bringing home a new pet. The biggest triggers for sadness in a dog, though, are loss, such as the loss of an owner or the loss of a fellow pet. Dogs experience mourning periods just like people do; when your dog loses a loved one, he enters a state of grief that may include lethargy, reclusiveness, passivity and refusal to eat. If your dog is acting depressed, consider seeing a veterinarian, who may be able to recommend a course of treatment and/or medication that can help cheer him up.