My dog who is 2, whom we have had since he was 7 weeks old, has started behaving very differently the last 2 months. He was socialized with aminals and people early on, never growled, bite, etc. Was never aggressive, except with food which we have worked on him with. Recently he has started gettin tense around people, even people he has seen many many times, such as my 17 year old neice. Now sometimes, he comes up and puts his head in my lap and I will pet him and he is fine. Sometimes he does the SAMe thing and I pet him and his fur goes up and he starts a low growl. Last night, he was enjoying me pietting hom, rolling for me to scratch his belly, sitting up letting me scratch his ears when out of nowhere he nipped my finger. He is a Lab/Border Collie which I never thought to be agressive in nature. He is growling at all people that don't live at our house, regardless of how many times he has seen them, ie. Our Landlord, whom he has grown up around since they are right next door! People that have been around him everyday since he was a pup and has even nipped at them! What could be going on with him? He was never aggressive with anybody ever until now! Any ideas on how to stop this behavior now?