Like many people in late middle age, I find myself wondering if my next dog will be different. I have long shared my life with retrievers β active, field-bred dogs who throw themselves with joy into every situation (and every puddle of water bigger than a dinner plate). Their boisterousness is infectious: My dogs make me happy. But in 10 years, or 15, or 20, will I be up to the demands of dogs like these? Or will a nice, quiet little spaniel be the dog I need? My answer may be different than others, even among people of similar...