Many breeds are by nature reserved with strangers. Dogs will read your body language and usually if you are welcoming, they will accept a stranger although they may never be best pals. Train your dog to sit quietly. Ask someone walk up to you or knock on your door, treats in hand. The person should bend down to a dog's level and either offer the treat with a hand or place it on the floor or ground. Do this several times with whomever you want your dog to be friendly towards. Your dog will soon associate the treats with that person and will become friends. While there are some breeds that will never trust strangers, there are also some people you might not want your dog to be friendly towards.