Recently, our four year old, male German Shepherd has become increasingly aggressive towards me. This seems to be happening mainly when my boyfriend leaves the house and it is just me and the dog. He has been growling at me and his back hair goes straight up when I simply pet him, or even sometimes when I just walk towards him. But then, once my boyfriend comes home, he will be right back to his loving self, follow me around, lay on me, bring me toys, and cuddle up right next to me on the couch. I am looking for any and all advice and/or recommendations for trainers in the area who might be able to help. Thanks in advance!