Weba dog wagging its tail is commonly perceived as happy. A behaviourist explained that assessing our pets' moods is more complicated. Webwhat does it mean when a dog wags its tail? We often misinterpret the wag of a dog’s tail, van haaften added. It’s more than just a meter of happiness. Webtail wagging is the behavior of the dog observed as its tail moves back and forth in the same plane. Within canidae, specifically canis lupus familiaris, the tail plays multiple.