By T. J. Dunn, Jr., DVM
The following essay is based upon thirty years of personal experiences working with dogs, cats and their caretakers. It is not intended to be a scholarly dissertation of psychological, sociological, or ethical foundations for behavioral modification. The views expressed here are my opinions ... you may have a different opinion based upon YOUR life experiences. You are welcome to and I will respect your opinion about this very difficult and emotionally charged topic.
While reading this essay please keep in mind that EVERY case of fear/aggression in dogs (and cats) is unique. No two animals or situations are exactly alike. Nevertheless certain predictable patterns are recognizable, and good judgment based upon informed and thoughtful introspection will lead you to your own best answers.
Aggressive behavior in dogs (and cats) can, unfortunately, be a source of conflict for humans. A certain percentage of pets will display aggressive behavior toward their owners/caretakers or other humans.
In the canine the fear and aggression occasionally seems to "come on out of the blue" but more often is triggered by getting into the dog's "space" or protective territory. This unsocial behavior, while it may be "normal" if the dog (or cat) were interacting with another animal to defend territory or signal "leave me alone", can be dangerous to people. Cats in this fear/aggression mode will bite and scratch...sometimes really terrorizing the owners. And dogs, with eyes glazed, teeth bared and with fearful barking and growling, will back owners into a corner or up onto a kitchen counter! In dogs this is often referred to as rage syndrome and can be a very shocking event for the owner (and I suspect, for the dog as well).
In the feline the aggressive mode may come upon the cat for unknown reasons. The cat will seem to be in a play mode, then the playing turns to more serious stalking, with ears held back and back arched, and often they will growl softly. You can see the fear/anger in their eyes. Or the behavior starts out while the cat is being gently stroked by the owner and the cat begins to become annoyed, then more defensive, then outright aggressive to the innocent owner.
The only way I know to defuse the aggression is to leave the pet's area -- just get out of eyesight. Trying to calm the dog (or cat), or restraining and disciplining him will simply make your pet even more fearful and aggressive.
What is the cause of this aggressive/anger state? It probably stems from very early personality/behavioral development experiences in the pet's life. Events such as deliberate abuse, accidental trauma from objects falling on the pet, scary stimuli such as thunder and lightning, or other animals frightening the puppy (or kitty) may make a permanent impression on it regarding the world around it.
More aggressive littermates can have detrimental effects, too. The critical age range that these events permanently make their impressions generally is from about four to twelve weeks of age; whatever is programmed into the brain's "personality structure" during that timespan will then be set for life.
As we all know, there are humans with personality disorders -- and outright sociopaths who are a danger to others. So it is in the dog and cat world. And as difficult as it is to "pacify" the behavior of maladjusted humans who have the benefit of counseling, therapy and medications, and the love and sympathy of family and friends, much more so is the difficulty in modifying the behavior of dogs and cats who pose a threat to their caretakers.