A family dog adds an extra dimension to a portrait. And, after all, they are part of the family. A few treats keeps their attention, although I learned the hard way about putting salami sticks atop my lens (more on that in a future post!)
A dog relaxes people- and gives them a focus other than the camera pointing at them. The smiles come naturally. So when people ask if they can bring their dog, I always say OF COURSE!