Possible Factors Contributing to Behavior Problems:
Lack of socialization - The socialization period is between the ages of 3 and 12 weeks for dogs, and between 3 and 9 weeks for cats. It is during this time that they learn what is "normal." Anything not experienced during this period tends to produce avoidance or fear responses later.
Traumatic experience - Just like humans, pets can have traumatic experiences that shape future behavior and decisions. This is especially evident during their sensitive learning periods.
Ontogenic component - The position of puppies and kittens in relation to littermates in utero, how much nutrition they receive, stress of the mother, and their environment immediately following birth can shape the behavioral health of a pet for the rest of their lives.
Genetic component - If the parents demonstrated behavioral abnormalities, the puppies and kittens are likely to as well. Additionally, early onset of fear/anxiety disorders is strongly correlated to family predispositions.
Multiple homes - Pets that have had multiple homes may be predisposed to developing separation anxiety and other anxiety disorders.
Compulsion training - Training techniques that rely on corrections, force, fear, or other punishment-type tactics will cause fear and anxiety, thereby increasing the risk of behavior disorders. Dominance hierarchies do not exist between different species, and therefore should not be used as a driving force for modern, science-based training methods.
Pain or medical component - Especially in the case of older dogs and cats, pain can be a large contributing factor for behavior disorders. Pain increases irritability and, therefore, aggression.
Exhaustion - When pets that are prone to anxiety or aggression become extremely tired, they tend to be more irritable. This could be the case after a very active day, a lot of playing, or if any separation anxiety is not well controlled. By the evening, these pets are simply exhausted and don't want to be bothered.