Like humans, dogs do occasionally suffer from the bouts of depression. They get mopey, lethargic, stop eating, and drink only minimal amounts of water, stop wanting to play, and even lose drastic amounts of weight.
What can cause this in pets?
Grief, change of scenery, or sometimes it is a chemical imbalance, needing medication to correct it. Even the weather can adversely affect a normally happy dog. Or your health.
Once you and your vet have determined that depression is the cause, there are a few options. One is medicated therapy. That would be Prozac for dogs, or other veterinarian recommended anti-depressants. An increase in your activity level, or at least your dog’s will also likely be called for. In cases of grief, playdates with other dogs or Doggy Daycare would definitely be worth looking into. If you are ready, you may even consider getting another dog.