I have a 3 year old male Mastiff that is neutered, and in the last couple of months has been aggressive towards other dogs. He has always had a problem with other males but last night he went after our friends female dog that has always come over, as soon as I let him out he ran straight for her. Both my Mastiff and St Bernard got along great with her before last night, they have never had ANY problems. I am going to start more training with him, he did go through obedience class and usually listens pretty well. I was just wondering if anyone knows of any medical reasons Diesel would start acting this way. I am bringing him to the vets this week, but any input would be appreciated. Also he has this obsession with his logs that he plays with, it seems like he gets stressed out and NEEDS to go to his log. If he is inside its his ball and he shakes it back and forth so hard until he calms down. If anyone else sees these traits in their mastiffs let me know. I am very desperate to fix the aggression, he is my baby and I can't face what will happen to him if he doesn't change