-Why Dogs Wag Their Tail-
Tail wagging it's all about your dog's body language
Tail Wagging Is Telling You Something.
It's a metaphor that means something is controlling your life other than your brain. Ordinarily, a dog will wag HIS tail when he is happy, hang it low when he is sad or in trouble or hold it high when he is confident. Figuratively speaking, if the tail is wagging the dog then it is controlling him..Not the other way around.
Actually a dog wags their tail in different ways. If a dog keeps their tail high and wags it, they are showing a sign of dominance. The bigger dog usually does this because they think they are in control! If a dog only wags the tip of its tail it means that it's unsure and/or nervous. The most popular type of tail wagging is when a dog wags his tail straight out, this means they are excited and happy about something.
In addition, If a dog is holding its tail up and waving only the tip, you should approach with care, watching for other signs like the position of the ears or growling. If a dog has its tail between its legs, however, it indicates submission. Frightened or nervous dogs may wag their tails stiffly between their legs. (Article by (AKC.org)