[rommel's mom]

When we leave Rommel in the car and someone comes up to it he will put his head out the window and lick people when they approach. But if the person puts their hand in the car he will growl and snap at them. He did this even to my husband but there was another person standing next to him, we don't know if he was confused if it was the other person that was reaching in or not. What I need to know if this is an allowed behavor or are we in trouble?
Also what makes a dog lick everone in sight one minute but another peson comes up and he growls at them. As you can tell I'm new with Rottweiler's and could use all the input I can ger. We are already dealing with him getting very upset in the car when other dogs come running and barking at the car. he proceeds to lung toward the window at them. He also get annoyed when we camp and while we are walking with him other dogs bark and approach him. He barks and lungs at them on his leash. Sorry got so long winded, thanks for the advice in advance.