Anxiety paired with frustration are the major triggers for leash aggressive dogs. If you have a reactive dog, then it is more than likely he is a lead puller, that is intensely focused on everything else but YOU.
Walking the anxious dog
Anxiety------> held back on leash------> triggers opposition reflex response------> triggers frustration = AGGRESSION
Let's look at this issue from a human perspective.
You have an important meeting to attend. You are running late because you have misplaced some important documentation that you must take. You are rushing around the house checking all the places you feel you may have left it. You are running more and more late... You are becoming more and more anxious, feeling you are not going to make your important appointment on time.
Finally, you find the documentation, you look at your watch... running 15 mins late... You rush out to the car overly anxious to get to your appointment on time. You start the car and begin heading out on your journey to the meeting, anxious because you are running late. You are on your way...You turn onto the major highway, and suddenly you are caught up in slow moving traffic... anxiety increases as you are crawling along at 10 kph. Anxiety continues to increase as you realise you are going to be even later, due to crawling along at a snails pace. You look at the clock in the car, triggering even stronger anxiety. You look at the cars in front of you crawling along. Your anxiety now triggers frustration. Frustration builds, and you start cursing the car in front of you, or the road works up ahead. You may even get snappy at a person sitting in the car with you. As frustration increases above a certain level, it starts to transform in to anger (aggression).
As humans, we have the ability to reason, and therefore keep our emotions in check. We can focus on calming ourself if we feel our emotions getting out of control. However, if we allow them to go to far, we even have trouble snapping out of that state.. Thats why we have to catch our emotions early, before they get out of control, and therefore start controlling our actions.
However, some that aren't in control of their emotions, turn a situation like the above example into road rage, to release their pent up anxiety, frustration and anger... Dogs also don't have the ability to consciously work on calming themselves, they just react on their current emotional state, much like a driver experiencing road rage. And to make matters worse for your dog, being restrained by the leash triggers opposition reflex. Which means the dog strains against the tension on the leash which the dog cannot consciously control, which in turn elevates the dogs current emotional state, anxiety and frustration. Even frustration in humans is triggered by opposition reflex response... as its resisting any type of force that is holding us back from moving forward.
A dog that is anxious on a walk and is frustrated due to being held back on a leash is no different to our human running late for that important appointment and caught up in slow moving traffic. However for the dog, unlike a human, it isn't able to see that its emotional state is getting out of control, and also cannot consciously focus on calming itself.
From the dogs perspective
We go to the cupboard and get our dogs leash. This action instantly triggers our dog to get overly excited and anxious about going for a walk. We put the leash on the dog. The dog is now straining at the leash, dragging us to the door. Anxiety is increasing waiting for the door to open. Finally the door is open, and the dog is off, totally focused on where it wants to go, full of anxiety and straining on the lead, dragging its owner with them. No different to our driver running late for that important appointment. The dog is straining on the lead pulling the owner down the street. The dog now sees another dog, and this triggers the dog to want to socialise. The dog now starts getting frustrated, because it can't get to where it wants to go due to being held back on the leash.. Not unlike our driver caught up in slow moving traffic (that is his restrictive leash holding him back). Frustration builds in the dog. Finally the frustration builds to a level that it triggers aggression (rage).. Road rage!
Classical conditioning is very strong at conditioning behaviours and involuntary emotional states. So after a few walks as per the example above, we through classical conditioning end up with a dog that is triggered into aggression every time it sees a dog on a walk. It was never that the dog was originally aggressive towards other dogs, it was originally triggered by the dogs elevated emotional state (anxiety----> frustration) and wanting to socialise. No different to a person that cannot control their emotions and experiences road rage. As Pavlov observed, he could elicit involuntary salivating in dogs with just a bell ring. We can also elicit anxiety in a dog through classical conditioning, also frustration, and even aggression.
So if you have an anxious lead puller and your dog is leash aggressive, then you will always have a dog that can be triggered in to an aggressive reaction. It's no good just punishing (correcting) your dog for its reaction, we need to work on why the aggression is triggered in the first place. We need to first focus on the dogs anxiety levels before and during the walk. We need to stop triggering opposition reflex response by conditioning calm lose leash walking. By keeping our dogs anxiety levels down to a manageable level, the dogs reactions will not be as strong, and therefore easier for us to work through. For example, if your dogs anxiety and frustration levels are at a 8/10 during the walk, then its frustration level will elevate above that when triggered.. so it has no place to go but a 9/10 or higher, and therefore we have a dog that is in an emotional state that is totally out of control (road rage). If we get our dogs anxiety level down to a 3/10 or lower on walks, then its frustration levels when triggered may only initially go to a 4/10, which therefore gives us time to correct the dog before its emotional state goes through the roof and triggers strong aggression, and therefore much easier for the dog to snap out of. We can correct the dogs behaviour by either positive punishment or negative reinforcement, way before it's emotional state goes way over threshold, and therefore allowing us to lower the dogs anxiety back to a more manageable level.
Anxiety is the biggest issue facing dog owners and is the number one cause for MOST behavioural issues...