When I first started my dog walking/pet sitting business in Mount Pleasant, SC, I think one of the most useful pieces of information that I received from experts was in regards to a wagging tail. I was one of those pet parents that always assumed a wagging tail meant a happy dog. That is just not the case. As an example, an aggressive dog could be wagging its tail because it’s about to act aggressively and it thinks it’s doing a good thing. It’s always best to ask the dog owner about the dog’s temperament before approaching a dog and certainly if you have another dog with you.
I would encourage a quick read of this article from VCA hospital and written by a DVM. https://vcahospitals.com/know-your-pet/interpreting-tail-wags-in-dogs