Not to frown upon larger dogs (my favourite is the Pyreneese (spelling?) shepherd), but they do often have signifigantly shorter life spans in many cases. Depending on what it does, how it's bred etc, a healthy BIG dog has a life expectancy of between 8-12 years, with the last 2 being happy, but not overly mobile. The way our vet described it once was, most dogs are built the same, any specific breeds will have a shorter lifespan on average, and most dogs have "average" sized organs - IE heart etc, so a lab will have the same sized heart and a Great dane, and the great dane will naturally put more stress of it's heart and other organs. There are of course exceptions to this rule, but something to be aware of, as once your dog's been with you for ten years, it hurts to have one collapse in the back yard or front hall and not have any way of helping it. That said, often they'll die because they've had a good life and have lived it well, so there are merits to any breed.