Is there a way to stop this behaviour? It's been quite annoying because every time someone is at the door, my Shepherd becomes very protective and won't stop barking. He wasn't like this before, it's only been happening for about 2 months. What can I do? He is 2 years old. Occasional barking at stranges in the street also takes place but generally it's a problem when someone visits us. Thanks in advance!

Have you tried to not open the door till he calms down? Saying a firm NO at the same time so that he realizes behaving like that isn't acceptbale?