Dogs can feel depression for a variety of reasons, but one of the more common causes is the loss of a close dog friend or the loss of their human. Other reasons might be changes to their routine, their humans going away from them on vacation, or illness.
Surprisingly, dog depression symptoms are very similar to human depression symptoms. Some common symptoms to look for are: moping around, lethargy, withdrawal from other dogs and humans, eating too much or too little, little to no interest in things that once excited him (walks or play), and sleeping more or less than normal.
There is no quick fix with dog depression. If you are not aware of the cause of the depression, it's important to see your vet to look for an illness or chemical reasons. If you know the reason, there are some things that can be done to help your pup through. Lots of positive reinforcement is important during this time. Providing time for encouraged play, especially with other dogs if the loss of another pup caused the depression, is a great idea. If your vet feels it's chemically driven depression, meds are available, but natural solutions are encouraged first.