Good lesson to remember: pets don't have purely psychological mood problems, at least not independently (they may react to the owner's good/bad mood and adopt the same mood, but they won't all of a sudden feel "the blues" when there is no external or physical reason for it). If your pet suddenly starts being sad or whiny, and nothing bad happened around him (wasn't yelled by the owner, bit by another dog, witnessed a fight between owners, etc.), that means there's an immediate physical problem (hungry, cold, in pain, nausea, etc.). Either way it won't be purely psychological. Trying to "cheer him up" without addressing that problem won't work, and can only lead to a result similarly to the one OP described.