Dogs communicate with each through body language, but most people do not understand or look closely at their dog's canine body language. When a dog continually tries to communicate and is not understood - they can escalate to things that do work. And work quickly. Growling... and then biting (if we scold them for growling, biting will be even more probable). Now these behaviors aren't terrible in dog/dog communication. Dogs growl at each other to communicate things... and it isn't a deal breaker in their relationship. They'll even get into scuffles, and the friendship picks up a few minutes later. But biting humans is a no no, and millions of dogs a year are euthanized for biting humans. Please read this important article by Kari Bastyr of Waggin Train in Denver to understand biting and growling in your dog, and how to manage your dog to set them up for success. http://www.wagandtrain.com/dog-training-articles/my-dog-is-such-a-sweetheart-but-he-bites-people-2/