Hi! I'm new here. I have a 7 y/o Rat Terrier mix. We adopted him 1 yr ago. We don't know alot about him except he was owned by an elderly lady that went to a nursing home. Whenever someone comes to the door or he sees another dog, he tries to bite and nip and makes horrible snarling noises. He's fine once the person comes in. How can I help him to not be so scared?