A moving message
Dogs use tail movement and position to convey information, including in encounters with people and other dogs. A wag does not automatically mean happiness.
A 2024 review by Silvia Leonetti and colleagues links wagging with both positive and negative arousal. The rest of the body and the situation matter too.
The whole dog matters
Picture a tail as one part of a conversation. Watching that part alone is like reading a message with most of the words covered.
The researchers describe different patterns of movement and carriage, including low-held wagging associated with appeasement. One simple wagging-equals-friendly rule throws away useful detail.
A question still open
Why do domestic dogs wag so much? The review offers possibilities connected with domestication: wagging may have accompanied selection for other traits, or people may have favoured rhythmic wagging itself.
Those are hypotheses to test, not a solved origin story. For another familiar canine puzzle, see why dogs tilt their heads.