How a Dog perceive people's fear: One very impressive thing that characterizes dogs is their extraordinary ability to perceive things; they can without any major effort perceive when a human fears them. It has been said that they perceive people because the organism of the person sets off a large amount of adrenaline that the dog can smell. However, when a person is afraid they tend to move a certain way, make certain facial expressions, and their eyes look worried, and the dog is able to perceive and capture these signals. Dogs are also able to perceive the good or bad intentions of some people and it is not easy to deceive them even when the person seems to be gentle and nice; dogs are able to notice the wrong intentions of a person.
Obviously this doesn't mean that they see a person's bad side when they bark at you. This extraordinary ability comes out in special circumstances and it is usually in the case of dogs
that have aggressive characters. It's pretty obvious that dogs
perceive the honest feelings of a person and that they feel empathy with people they feel like them. What is not normal is when a female dog has been living amongst humans and then suddenly becomes aggressive towards the owner if he or she tries to touch one of her puppies. This type of behavior means the dog has not reached a complete level of confidence in her owners and it makes her react instinctively to protect her puppies. You must also take into account that there are many different breeds which have been genetically conditioned to act as predators. This factor is not always known by people that bring them into their homes. You must take into account that there are certain breeds that are not intended to live in the city or in a small apartment etc, and that can cause the dog to respond to his natural predator characteristics because he is not in his natural environment.