When someone new arrives on the scene, dogs are alert. Some may even act aggressively or defensively. Dogs are animals and all animals are wired in a way that supports the best chances of survival.
If you watch nature shows, you have probably seen wolf packs interact. They respond swiftly to the arrival of a strange wolf and are instantly on their guard for the intentions of the newcomer. New animals could represent a threat to the group as an indirect competitor for resources or a direct threat through attack.
You are your dog’s family pack. When someone approaches you, a similar scenario unfolds for your dog. Who is this person and what do they want? Are they going to attack me or my person? Maybe they are going to compete with me for my position in my family group and try to oust me. Maybe they are planning to draw the attention of my person onto them.
You represent both a resource (since you meet your dog’s needs, emotionally and physically) and a family member. Both must be defended.
If you know that the stranger has good intentions and you know your dog, it might be a good idea to make the new person into a resource provider too. Arm him/her with a pocket full of your dog’s favorite treats. Make sure your dog has trained behaviors that he knows will elicit a good response from you, like sitting on command. Repeat this training so often that your dog knows what to do without really having to think about it. Then when the newcomer approaches, ask your dog to perform the trained behavior and reward it. Then have your friend ask and reward also.
If you like the new person and you take time to let your dog see that you do not feel threatened, she should become more comfortable too. Soon the “stranger” won’t be strange at all and your dog won’t be suspicious anymore either.