Why is my dog so aggressive to other dogs?  This can usually be diagnosed with a detailed history: no play ever, hereditary, mother was sick or a guarder, or over socialized with aggressive or rough playing dogs.

Cycle of On-Leash Aggression (created problem from humans), as described in the Culture Clash, by Jean Donaldson, “The Bully dog” is often kept away from other dogs for long periods of time, he is usually rude with crude behavior brought on by a super motivated greeting as a result of deprivation when meeting other dogs, and has poor social skills.  The owner is alarmed by intensity and tightens the leash and get’s too excited or nervous when interactions occur.  High arousal, lack of social skills, scuffles with defensive dogs can occur.  Barrier frustration such as windows, fences and leashes can increase the dogs frustration which makes you want to “correct” the behavior,  which = punishment  which = more Aggression = total isolation.

I believe dogs need time to express their intentions before they greet unknown dogs.  Personalities among dogs differ as much as a classroom full of kindergarteners, therefore, expecting your dog to like every dog they meet is not that simple. Some dogs are very soft and have appropriate greetings, these are the dogs who are able to visit the beach and off leash parks without incident. 

 
Helping your dog greet new dogs much slower will give your dog important and necessary information about the other dogs intentions.  To the left, you see the brown dog in the middle of this pack at a local park.  He is standing quite still with head lowered, visible tension in his jaw, mouth closed, low tail, ears back and a his hair beginning to stand up on his back.  He is very uncomfortable about being so close to a strange dog and was called away quickly to avoid any conflict.  This particular dog’s behavior tells us humans that he needs a much slower greeting with new dogs.
 
In this photo to the left , this beautiful girl has just seen a new dog and is reading the other dogs intentions and clearly expressing hers as well.  Note the open mouth and soft eyes, lowered tail which is in motion and she is beginning to offer a play bow.  While she is expressing intentions that she does want to greet the new dog, she is very excited and the other dog is a bit alarmed by her intense need to visit.  After about 30 minutes of walking near each other, this girl and the other dog became play mates as you will see in the video below. 
If you have a new puppy, please keep him/her safe and find nice friendly dogs to socialize with.  Your dogs friends will influence his/her behavior! Just like you were influenced by those you visited with as an adolescent.  So, know who your dogs friends are and watch for signs of fair play between the two and you let’s try to prevent aggression from spreading.