Dogs know how we feel just from our body language. I think they can sense a chemical change too. I know they can sense fear. As long as the unwanted visitors just knock, barking is good enough for me. But any funny business, and things should escalate with the dog's behavior. I have a "no soliciting" sign on my door. I don't know if people are just blind, or they ignore the sign. Maybe they don't know what that means. If someone comes to my door, you cannot carry on a conversation there anyway. The barking is so loud, it is useless. My son's dog is teaching my dog to be a watch dog on the property. (her genes would have taken over, but now she is receiving lessons from the old dog) Right now she is sucking up the A/C in the house. It was 105 deg out there, with 112 heat index. Hot as a desert like hell, to me. I bring the two dogs in during the hot time of the day. They take a nap then mostly. Then before supper, I let them back out. About 7pm they are in for the rest of the time. The old dog looks sort of like a pit. But he isn't. Most people think he is, and they are afraid of him. He is 1/2 Boston Terrier. His mom was a German Pincher. So he was a very gamey little dog, weighs 32 pounds. (solid muscle)