aggressive behavior in 5 month old puppy

Question
My 5 month old golden retriever not neutered male, who just recentley went thru obedience training and is doing well,curled his lips and lunged at my boyfriend who was lying on the floor petting him. My puppy did have a washcloth in his mouth, but boyfriend did not try to take it away. This was totally unprovoked. What should I have done in this situation and how can I correct it? Thank you for your expert advise...most appreciative.

Answer
Julie, what I think you're seeing here is the beginning of resource guarding.  Please read over this excellent article, follow the suggestions and if you are still having issues, you will need professional help in-person.

http://www.diamondsintheruff.com/RG.html

You should also teach your dog to drop things on cue.  Here is my favorite technique:  http://www.youtube.com/watch?v=ndTiVOCNY4M

Please let me know if you have follow up questions after reviewing the material. Good luck.