Our local County Extension Agent once told me that to use the Great Pyrenees dogs effectively, they HAD to be raised with the sheep. As an experienced ( life long ) sheep man, he knew what he was talking about, I think! He said that you put the puppy in with the sheep ( I think he said on a leash, with lambs and ewes, but with a little protection, like wood piles, crates, pallets, or whatever ). The ewes butt them around and beat up on them pretty good, but they begin to think they are a sheep. ( Weaning age, of course!) From then on, evidently, they protect the flock from intruders!
Mark