There are two types of aggression: attack and swoop, that are natural and unnatural aggression.
Natural aggression
This is not aggression just for its sake or proving power, since that is unnatural to occur within a pack. Anyhow, in certain circumstances natural call for attack may occur, whether it is the same species or different one. Proper upbringing will put your dog's natural aggression under control. Otherwise, learned aggression will develop.
Circumstances under which natural aggression may occur:
a) Hunt: In order to survive in wilderness they must attack and hunt other animals. In domestic dogs such urges must be under control. The owner must not let its dog to swoop on other animals or humans.
b) Hierarchy: They will fight in order to establish rank within a pack. It is natural to display aggression only until they win the argument. Everything more than this is unnatural. This is why when owners stay put at the park is potential risk for wakening dogs' natural instincts to establish hierarchy since dogs tend to experience themselves and others as one pack.
c) Territory: They are territorial beings. It is in their nature to defend space they live in and to react if someone enters it. Wolves and other dogs in wilderness defend their forests and meadows, while domestic ones defend their yard, apartment, car... Some are less, others more territorial. It must not be allowed that your dog treats any space it ends up at as its own possession.
d) Food: In wilderness, dogs that are on top of hierarchy first have access to food, then the others. Weakest members of a pack eat last. This is why it is important for an owner to show that he/she is the leader. Do not feed your dog outside or other owned dogs since it can waken instinct to fight for food.
e) Mating: Males come into a conflict because of the female and such fights end in serious injuries only if both males are similar in dominance and strength. It is essential to avoid meeting of two males and female in heat because conflict will be inevitable. Some females have distinct scent throughout the year which can also trigger this natural instinct.
f) Litter: Bitches protect their offspring. They can show enormous amount of aggression if they feel that someone is a threat to cubs. They will vigorously bite if someone attacks them. As they grow the amount of aggression will decrease. This is why you should not approach a bitch that shows clear signs that you are unwanted.
g) Pack: Dogs will very often defend other members of a pack. Especially stronger and more dominant ones will defend weaker and frightened ones. This is why you should not pay attention when your dog approaches owner with two dogs.
h) Fear: Dangerous situation with no way out will make a dog run away or attack. Do not let your dog approach another dog that looks frightened.
Unnatural aggression
It will occur in domestic dogs "thanks" to humans. This human input can be divergent or combined. One occurs due to controlled breeding, other is learned aggression.
1) Controlled breeding: Creating breeds with certain genes that display aggressive character towards humans, dogs, some other animals. These dogs were created by humans with aim to be this way.
2) Learned: This type of aggression is one of the most common behavioural problems which occurs due to improper upbringing. As well as genetically controlled (mixing breeds in order to create aggressive dogs), this one can also be displayed towards humans, dogs and other animals.
Limitless aggression
Aggression can escalate so much that will make the dog enter a state in which it will be completely out of control. It will have intention to attack immediately. This will not be the case of defending territory, female, food... but the case of constant aggressive state of mind. It will attack its target until it gets tired, or has no energy left; it will have a need to catch something in order to slaughter. Humans are responsible for such state of a dog. It will not occur out of a sudden, but gradually due to improper upbringing.
Any dog can develop limitless aggression. But the strength of a dog-attacker, as well as strength and size of a victim will determine the amount of damage.
Although any dog can develop it, it is most common with breeds created to fight, service dogs or the ones that hunt other animals, since their predispositions to turn this way are stronger than with others. But no matter which dog is in question – no dog should be in such state. If the dog is in such state – problem should be solved immediately!