Probably the most common type of aggression, which meets a dog owner, especially the owner of a male, is the intraspecific aggression directed to the other males (in the other words - the aggression of dominance).
Males, depending on the breed: small breeds at the age of 6-8 months, large - at 8-12 months, start to mature, to realize themselves an adults. They actively mark their territory, they begin to review their hierarchical status, and starting from this time most of the encountered males are perceived by your male dog as potential enemies. Adulthood, depending on the breed, starts approximately at 1,5-3 years.
It is the most likely that the fight will happen between the dogs, that are roughly in the same size and in the same age. Large dog does not perceive a small one as a serious competitor, and a small dog is not going for a suicide, to be eligible for re-division of statuses with a large dog (although many small breeds of dogs are quite eager to fight - the owner should remember about this).
The hierarchical status of a dog may decrease with the age or due to illness. As a rule, his applications for the leadership your dog will demonstrate by a confident posture, direct view, the applicant for a higher place will be the first to approach the second dog, which is waiting for his actions, standing on the ground, and the second dog can also sit or even lie down. The dog, which has a higher status can push the younger dog by body, put his paw or head to the younger dog's withers, etc. If the second dog meekly accept these claims to the leadership, the conflict will not develop. Otherwise, there may happen a clash, but first anyway there will be the demonstration of a serious intentions, and only then - the fight by itself.
As it was already mentioned above, very often dogs of a small breeds act as provocateurs, barking at all encountered dogs. Even if initially the larger dog may ignore the small attacker, the giant's patience sooner or later may end up, and even a slightest demonstration of intentions of a large dog can end up badly for the little provocateur.
As a rule, owners themselves unknowingly reinforce the undesirable behaviors of their small dogs. You take your small doggie on your hands - it elevates the dog in both: the literal and figurative senses. In this case the small dog feels the support of her owner: we are together, we are a pack. The owners are trying to calm down their little fighter with the sweet words and with stroking and sometimes whispering in his ear. But this is, in essence, a supportive behavior: with such intonations, and often with almost the same words the dog is praised for the desired behavior. Compare: "Well done, Bonny! Good dog!" And little Bonny is stroked and prized for a properly executed command. And this: "It's all right, Bonny, calm down." And Bonny is stroked again, while he is sitting on the owners hands and barking at the other dog. In the same time the owner of a bigger dog is taking his pooch away and the small sized aggressor believes that what happened here - is his well-deserved victory. Also he perceives his behavior as the right one, the one, that his owners like, otherwise they would not stroke and praise him of course, and next time with even greater enthusiasm the little fighter will bark at the other dogs.
As a rule, after two dogs have sorted out their relationship, they do not return to this question for a very long time (sometimes years). If the male shows aggression to the already familiar males or females every time he goes for a walk, most likely it means that relationships are not clear. Otherwise, may be it is, for example, a demonstration of territorial aggression or fear aggression.
A similar behavior may also occur in females. But here are some pitfalls. Unlike males, which will install the ranks and over several years will not get back to this issue, the position of a non-sterilized females depends on the heat. During this period, the status of a dog rises, so pretty often you can see that other females demonstrate a simulated sexual act with the bitch, that is in the heat. This usually is done in particular by those females who want to maintain their leading position, and thereby they show to the bitch in a heat, that they will not let her to raise up her position, reminding her place in the pack. On the other hand, a bitch in a heat also wants to improve her position in the pack, so there may be conflicts. Here the task of the owners is to avoid clashes and to support the natural leader of the pack.
Castration of male dogs is recommended to reduce this type of aggression, and the earlier the male dog will be castrated - the better, before the undesirable pattern of behavior will be fixed. But with bitches sterilization can not solve the problem, since on the contrary, the decline in female hormones and thus the increased level of the male hormones, leads sometimes to even more aggressive behavior. Especially, this trend can be traced in the young dogs, which were sterilized early. It should be noted of course, that the sterilization of the bitch, that is not seeking dominance, does not lead to any further aggressive behaviors. It is also important to remind that a healthy dog, that is not going to be used in breeding, is better to be neutered.
It is important to teach your dog to respond calmly to the opposing dogs. Contrary to the popular belief, adult dogs do not have a strong need in the games with fellow tribesmen. For the urban dog, the owner's family is a pack. It is important to be involved in your dog's life, to play with your dog, to socialize your dog properly and to give her (or him) enough amount of physical and mental work. If your dog meets other dogs on the walk, it is quite enough for them just to "say hello" to each other.
If your dog reacts aggressively to the other dogs, it is important to understand at which distance usually starts this manifestations of problematic behavior. Further, the owner should work on changing his dog's reaction to the opposing dog. The same technique is used to modify the behavior of dogs exhibiting aggression towards people, runners, cyclists, etc.
When meeting the "enemy", the owner must take his dog away to a distance at which the dog responds calmly. Then give your dog a command (for example "sit") and let the "enemy" to move away. Then your dog should be praised and rewarded a treat for the calm behavior. The same thing you should do every time, when you meet the other dogs on the walk. When the dog is showing a steady success, the distance can be gradually reduced. At each new stage of this training, the reducing of a distance may be about half a meter. The owner must be between his dog and the "enemy", making the potential communication between them more difficult. Behavior of the owner himself should be calm and confident. Of course, the owner should be the undisputed leader for his dog so that the dog adequately responded to his commands. Never force the events. By the end of this program of behavior modification, your dog should be able to pass the other dogs calmly without paying any special attention to them.