Question: Why do dog's tails wag when they are happy?

Rebecca Dewey answered on 22 Jun 2015:

A dog’s tail wagging is a form of communication, such as a human smiling, frowning, saying hello, waving their hand or nodding their head in recognition.

A dog wagging it’s tail can be many forms of communication (and may well be individual to that dog). Depending on the dog, and on the direction and speed of wagging, the dog could be telling you that it is happy and friendly, or alternatively that it is scared, insecure, challenging you to fight or warning you that it is about to bite you. There is research going on to try and classify these different wagging gestures into different categories so that we can understand animals better. For now, you are best off asking the owner if it is safe to pet their dog – they will probably have understood their own dog’s language!