What is on Leash Agression?

A reader submits: My dog meets and plays so well with other dogs at the dog park. But when he meets another dog on leash, he acts completely different. He has even attempted to bite a dog. What is wrong with him?
On leash aggression is a common behavior. A lot of dogs who are fine with other dogs when left to their own devices, can suddenly become aggressive when meeting on leash. In fact, the behavior is so common; I simply never
allow my dogs to meet other dogs this way.
Now, some readers may be thinking: If the owner knows their dog has on leash aggression, then THEY should prevent THEIR dog from meeting on leash. My dog’s an angel and this doesn’t apply to me. Unfortunately, that’s
where they’d be mistaken. Dogs communicate with one another through body language; primarily the use of ‘calming signals’. This language has at the root of it one beautiful purpose: To communicate peaceful intentions in order to avoid conflict.
When a dog is on leash however, his natural body language can be unintentionally inhibited by his handler. This is where we can start running into a communication breakdown. The improper application of tension on the leash or the use of harnesses that encourage pulling behavior can cause
your dog to posture against their restraint. This often looks to the other dog like an aggressive, forward advance and sadly, initiates the most altercations.
Now you can see how even the most happy go lucky dog, pulling enthusiastically in the direction of his potential new playmate is communicating ineffectively. The other, also restrained dog feels anxiety from being approached in this manner, but is unable to create the needed distance and will be much more likely to react. So, it isn’t that either dog is aggressive. Instead, one has been ‘backed into a corner’ and is merely protecting himself from a perceived threat. It is, on the head of the
handler who has unknowingly creating a situation between the two leashed dogs that would have never existed without human interference.