Why does my dog wag its tail when it’s happy to see me?

A dog uses its tail wagging, facial expressions, ear position, posture, and vocalization to communicate. Puppies start wagging their tails when they are about six or seven weeks old, as they begin to interact with their fellow littermates and humans. At this time, puppies begin using their tails as a means of communication and social interaction. Animal specialists believe that tail wagging is in the dog’s genes: since wolves (the ancestors of today’s domesticated dogs,) ran in packs, communication—whether by barking, growling, or wagging tails—was essential to survival. Researchers agree that a dog with a loosely wagging tail is usually “saying” that he is friendly or excited. However, a wagging tail is not always a sign of friendliness: an aggressive dog might hold its tail high and wag only the tip, while a submissive or scared dog is more likely to hold its tail low and wag it stiffly. Look for these clues in your own dog. If you ever approach anyone else’s, ask the owner for permission to pet the animal before making contact.

Reference: Britannica