we have a male 2 year old chol. lab. We have had him since he was 6 weeks old. We also have three other dogs, a male black lab 7 years old, female yellow lab 8 years old and an dog female (heard type dog border collie/Australian shepard). The border collie mix is an somewhat aggressive but been told that is in her blood line. She is the boss! Just recently the 2 year old male lab has started growling and wanting to fight the 7 year old male lab. The 7 year old male is about as gentle as a dog can get. He doesn't really bother anyone. What would cause the 2 year old to start doing this toward the 7 year old? What can we do to stop it now?