In the case of jealousy the mind of a dog works in almost an identical way to that of a human being. It wants the full attention and love of its owner whether the jealousy occurs only when another dog enters the home or when the beloved owner talks to another dog outside, or whether the jealousy is aimed at another person in the home. The same driving force is at the root of the evil in all of these cases: the intention of the dog to reign alone and supreme in his household.
The guarding instinct so prevalent in some breeds has its roots in the same sort of thing; a desire to let no one enter the precincts of his master or mistress. Jealousy nearly always takes the form of a show of viciousness toward the dog or person the animal is jealous of. Quite often it is a mild form of jealousy and only involves its bone, toy or the piece of rug that it is fond of. It jealously guards them and woe betide anyone trying to take that object away.
This jealousy is particularly pronounced when puppies are reared and kept in the household. As the puppy reaches the age of about three months the mother will begin to feel jealous as her maternal instinct fades and the time draws near for another heat. In spite of the attempt to treat both dogs equally and always to talk to both at the same time, feeding both at the same time and exercising both together, the jealousy continues to grow.
Correction works at first and then bit by bit grows less effective. In the dog's mind a usurper has entered the scene, and, as in the wild state, it is trying to turn the young out of the nest. As she fails to get rid of the now grown up pup, her temper gets worse and worse in the effort to dislodge the now adult and unwanted member of the household. She becomes more and more thwarted as her owner attempts to make the newcomer as welcome as the old established member. Often she will turn on her owner when he is trying to make peace, as if she were trying to impress an ignorant person that it was time the youngster went out into the world to fend for itself.
If you are a really good handler your training methods will be good enough to make both dogs obey the command "Leave" when they are in your presence. The danger lies in the times you leave the dogs together on their own, for the slightest boldness on the part of the youngster in approaching the older one's basket or toy, etc., will infuriate the older dog, and she will set on the youngster tooth and nail.