For the most part, I agree with what Bob's dad said. I do not think something triggered this behavior as much as it is just the dog maturing. Even the AKC recognizes the temperment of the breed and will immediately disqualify the dog for out and out viciousness but recognizes an aggressive and belligerent attitude as acceptable without being disqualified....

From the AKC site....
Viciousness: A dog that attacks or attempts to attack either the judge or its handler, is definitely vicious. An aggressive or belligerent attitude towards other dogs shall not be deemed viciousness.

Before someone says something in defense of the dog, let me be first. I think these dogs are very out going and loving and definitely smart and obedient (obedient being the operative word here)! They require a strong handler with clear objectives set in the training exercises. Training should be on going and not stop at the end of socialization or basic obedience. When someone comes to the house, the dog should be put in a firm and unquestioned down position so that he understands you make the decisions on who comes into the house and not him. Never use harsh punishment with this breed (or any breed in my opinion), only positive reinforcement. Using harsh punishment could defeat the purpose and make the dog more aggessive / territorial.

Dobes respond great to clicker training and are quick learners so my advice would be to get him back into training with a qualified instructor who uses positive training, preferrably clicker training and never give him anything unless he works for it first. You should start to see improvement soon.

However, at some point you are going to have to do specific desensitizing exercises for people coming to your house since this is the main problem. Ask someone not close to you but not afraid of your dog either to help out. This should not be done though until your dog has a firm and reliable down stay.