Correctly bred and reared dogs of the natural guarding breeds will guard you without being trained. Dogs such as GSD's or Rotts know when they need to do their job, but they need to be able to live among people most of the time. Think about police dogs, they have to do things like school visits one day and catch criminals the next. They must have an excellent temperament and a natural guarding instinct. If they are over aggressive then they are taken out of training programmes as they cannot be trusted in a normal environment.
None of my GSD's has ever been taught to guard, but on the rare occasion when they felt I was being threatened, they would put themselves between me and the 'threat' and bark if necessary, although just the look of them is enough for most people with dishonourable intentions.