Ursprünglich als Antwort auf diesen Artikel:
I think dogs are wagging their tail when they are agitated - this can be because of happiness or because of something else. But you will see the difference, if you know the dog, from the other signs. What is the rest of the body doing? Is it up straight? Is he ducking towards the ground? How do the eyes and the ears look?
You have to consider the whole body language not just the tail!
I don't believe that it is always a display of conflict, when they wag their tails. Maybe the movement stems from that background. Like the laughing of humans too. I once heard a theory (in a television documentary) that some scientists think that loughing with the mouth wide open - the big "ha, ha, ha" - comes from an expression of surprise or little shock - like the first moment when you hear the puch line of a really good joke and you hold your breath for a split second til you burst into a really good lough.
I think it's the same with the tail wagging. The excitement has to go somewhere and so often the whole body starts to move. If you reduce that movement to its most basic elements - it's the tail wagging.