Have had several large breed dogs over the years (currently have a Boerboel) and have come to the conclusion that there's no such thing as a guard dog that can be a pet or vise versa....you need to decide what you want the dog for...either pet or guard dog.... if you want a guard dog then you cannot expect that the dog will perform as a guard dog then be all cuddly to your family, children and their friends....same goes for the opposite, you cant expect a pet to be all friendly to the postman, milkman etc and then know that it's supposed to tear the legs af any knackers that arrive at your door.
Don't get me wrong if certain breeds are raised as pets they will protect your family out of natural instinct but you can't blame the dog if it all goes wrong....also you can raise certain breeds as pets and just the look of them will scare the shite outta most prople anyway
Just my 2 cents worth