nipping dane

Question
my Dane about 1 that is snapping at company that comes to my home. He will go up to them and ask them to pet him(by going and set down next to them and nudge them with his head)he will even let them pet him then with no warning no change in body at all he will bark and snap at them aggressively. I am stumped

Answer
Simple lack of environmental socialization.  He is not asking to be petted with his approach he is asserting himself and exploring his position in the new pack environment created when the new member enters.  With his age he is just becoming self confident enough to start this type of behavior.  The dog needs to understand this is an unacceptable behavior and be taught how to behave in this situation.
Please review this answer to another dog owner with a similar situation:
http://en.allexperts.com/q/Dog-Training-3333/2010/1/agression-around-new-people....  
Use the same process simply insert your situation.  You may use a muzzle to prevent any accidents at first. You may also want to purchase my booklet defining and using the ONE, TWO, GOOD DOG! protocol.  http://invincibleservices.com/html/one__two_good_dog_.html

If this doesn't provide you with an answer please feel free to call me.  Dr. Hogan 509-991-0385 until 10 pm pacific time