DDB are natural guard dogs..all the stuff about sunroom and leaving and reentering is IMO normal..this MAY have been his job on the reserve and even if it was not its still a natural instinct to watch and alert ( hes not attacking) hes letting you know something NOT NORMAL is happening., I also agree that Platz is correct in assuming somebody on that reserve is a heavyhanded SOB. Some here will disagree with this but on all my rescued Molossers I INSIST that newcomers meet them the 1st time in my house/business/property by kneeling down and slowly extending a hand palm up, and I am always there for that meet and greet..if the dog is young and a tad shy I usually tell the peson to pat his/her hand on ground to instigate play. I have found this lighthanded approach OVERTIME will let the dogs become better with people..I would also get this pup to some puppy classes ASAP to meet new strange people and to watch how other dogs act with their masters. You seem to have a good handle on whats going on so just take it slow..I BTW LOVE a dog who will work for food (rescues almost always do) I mix the food and toy play up pretty evenly and never tolerate any food or toy aggresion. BTW I also think BabiGirl is correct that this dog more than likely meet very few people indoors , so this is all new to him. Naming a puppy Cujo shows you the people were arse's IMHO