Some dogs are nervous and fearful. The question is why is this happening? Has your dog been abused. Or is your dog picking up queues from you? Do you act nervous or fearful around people or around strangers? Do you get tense when strangers are around? How do you interact with strangers? Try relaxing and encouraging your dog to be friendly. Make sure your dog has his personal space respected. Try and see if your dog is sensative to kids or loud noises or something in particular.