I have a similar situation with my dog Spike. But maybe it's the opposite problem? I mean, he is way protective as well. He barks and growls until he sees who's at the door. Then he is happy and wants to attack them with kisses.
I figured this was a breed norm though. If someone wanted to break in, they would change their mind quickly when they heard the big dog on the other side of the door going insane. Is this something that one would want to have looked at by a dog trainer or behaviorist? I do not believe he would hurt anyone, but you never know, if they were not invited in...
Know how people have been sued in the past because someone broke in and got attacked by the home-owner's dog? That seems extremely unfair.. Scary thought.