When a dog wags its tail high and arched slightly over its back, people think it is happy. Another common misconception is when a dog's tail is down, wagging slightly. People will say the dog's a little bit happy. But a broad tail wag really does mean a dog likes a person or another dog. Both, said Coren.
People have to read the other signs that the dog is giving off with body language.
A dog with its teeth bared and its tail straight out behind is snarling and spoiling for a fight. But a dog with its teeth bared and wagging its tail broadly is smiling. I was asked to help with an Irish Setter named Finnigan who was sent back to his breeder because he was too aggressive. He was leaping and snarling at visitors and other dogs. To people who do not understand dog language, that could have been a snarl, but actually it was a submissive grin which meant, 'It's OK, I'm not a threat.