Factors contributing to the likelihood of the development of dog aggression and fighting include (in no particular order):

• Lack of exercise, which contributes to the dog's high energy and unstable state of mind
• Lack of discipline
• Insecurity in the dog, which is often a result of a traumatic experience
• Breed pre-disposition to fight or be aggressive
• A non-existent pack leader, or non-dominant owner (for example, an owner may not establish him or herself as the leader and dominant one), which would result in the dog becoming the "owner" of its surroundings, and it establishing itself as the dominant leader - this would result in aggressive behaviour (submissive or non-dominant dogs do not let out aggression)
• Living as part of a multi-dog household
• Lack of proper exposure to other dogs during the critical socialization period
• Lack of exposure to other dogs in general, which would result in the dog being insecure and also, unsure how to socialize
• Early imprinting by an aggressive or nervous dam
• Being raised as part of a large litter (more than 5 puppies)
• A traumatic experience associated with other dogs, such as being attacked or having seen a fight
• Lack of abundant resources (food, water, sex, etc.) in the environment
• Thyroid malfunction or other medical conditions

identifying the problem

Dog aggression does not usually become a serious behavior in most dogs until they reach the age of 1.5 - 3 years. Prior to that age, most dogs show only the seeds of developing aggression, such as fear and/or nervousness around other dogs, displays of aggression only under certain circumstances (while on leash, in the presence of food, in the presence of the owner, etc.), or most commonly, over-the-top play behavior.

Play behavior such as tackling, chasing, mouthing, nipping, pawing, and wrestling are all normal canine behaviors that serve the evolutionary function of preparing the young dog for later combat and hunting. Young dogs that engage in excessive amounts of these behaviors are much more likely to develop dog aggression as they age.

Dog aggression should not be confused with human aggression, which is allowed in some breed standards as well, as long as the aggression springs from a desire to defend the dog's owner or territory.