Reactive dogs: understanding why your dog lunges and barks on leash

If your dog explodes into a barking, lunging frenzy every time they spot another dog on a walk, you are absolutely not alone. Leash reactivity is one of the most common issues I see in my work as a dog trainer — and it’s also one of the most misunderstood. Many owners come to me convinced their dog is aggressive, or broken, or just plain bad. In almost every case, that couldn’t be further from the truth.

Let’s unpack what’s actually going on — because understanding the why is the first step towards genuinely helping your dog.

Reactivity vs aggression: they’re not the same thing

Reactivity and aggression are often used interchangeably, but they describe very different things. Reactivity is an overreaction to a trigger — an intense, emotionally charged response that’s out of proportion to the actual threat. Aggression, on the other hand, is specifically about intent to harm.

Most reactive dogs are not trying to attack anything. They’re overwhelmed. They’re frightened, frustrated, or both — and the leash means they can’t do what their instincts are telling them to do, which is either get away or get closer to investigate. The result is a big, noisy display that looks alarming but is actually a dog screaming “I can’t cope with this!”

Why does reactivity happen?

There are a few common roots:

Fear: Many reactive dogs have had scary experiences with other dogs or people, or simply weren’t socialised enough as puppies to feel comfortable around them. When a scary thing appears, barking and lunging is their way of trying to make it go away.
Frustration: Some dogs are actually very social and desperately want to get to other dogs to play — but the leash stops them. This frustration can build into a frantic, reactive display even though the dog isn’t scared at all. These dogs often wag their tails madly while also going completely berserk.
Barrier frustration: Being restrained by a leash (or a fence) creates a particular kind of frustration that can push even a fairly calm dog towards reactive behaviour. The barrier itself is part of the problem.

The stress response — what’s happening inside your dog

When your dog sees their trigger, their nervous system fires up a stress response. Adrenaline floods their body. Their heart rate spikes. All that lovely training you’ve done can genuinely become inaccessible in that moment — the brain goes into survival mode, and thinking goes out the window.

This is important to understand because it means your dog isn’t choosing to ignore you when they react. They are physiologically incapable of responding normally in that state. Getting frustrated or telling your dog off isn’t going to help — they literally cannot process it.

After a reactive episode, it can take anywhere from 20 minutes to several hours for stress hormones to fully clear a dog’s system. If your dog has multiple triggers on one walk, that stress can stack up quickly.

Understanding threshold

Threshold is a concept that changes everything when it comes to reactive dog training. Your dog has a distance at which they can see their trigger and stay reasonably calm — this is called being “under threshold.” Once that trigger gets too close, or too many things happen at once, your dog goes “over threshold” and the reactive response takes over.

The goal of management is to keep your dog under threshold as much as possible. Every time your dog goes over threshold and has a full reactive episode, it reinforces the habit and makes the next one more likely. Every time they see a trigger and stay calm, you’re building a different pattern.

Management strategies that actually help

Management isn’t training, but it’s essential. It protects your dog from practising the behaviour while you work on it, and it makes walks actually survivable.

Increase distance: Cross the street, duck behind a parked car, turn around before your dog notices. More space equals less stress.
Choose quiet times and routes: Early morning walks, quiet streets, and parks at off-peak times can dramatically reduce how many triggers you encounter.
Watch your dog’s body language: Learn to spot the early warning signs — stiffening, staring, ears forward, weight shifting forward. If you intervene before the explosion, you’ve kept them under threshold.
Use a comfortable harness: A well-fitted harness gives you better control without the discomfort of pressure on the neck, which can actually make reactivity worse.

Counter-conditioning and desensitisation: the real work

Management keeps things manageable. Counter-conditioning and desensitisation (CC&D) is what actually changes how your dog feels about their triggers over time.

The idea is simple: you pair the sight of the trigger (at a safe distance, under threshold) with something your dog loves — usually high-value food. Trigger appears, great treats appear. Trigger disappears, treats stop. Over many, many repetitions, your dog starts to associate the scary or frustrating thing with something good. Their emotional response begins to shift.

This process takes time and consistency. There are no shortcuts. But it works — I’ve seen genuinely reactive dogs transform into dogs who can walk past other dogs calmly, because their owners put in the patient, steady work.

Why punishment makes reactivity worse

I want to address this directly, because it’s still very common advice — telling off a reactive dog, using a check chain, or using any kind of punishment when they react almost always makes things worse. Here’s why: if your dog is reacting out of fear, adding pain or fear on top of that deepens the negative association. If they were reacting out of frustration, punishment adds conflict and stress to an already charged situation. Either way, you’re not addressing the emotional root of the problem — you’re just suppressing the behaviour temporarily while making the underlying anxiety worse.

A dog who has been punished for reacting may stop barking — but the fear is still there. Sometimes these dogs become more dangerous because their warning signals have been trained away.

Realistic expectations

I always want to be honest with owners about this: reactive dog training is a long game. You are changing your dog’s emotional response to something that feels very threatening or overwhelming to them. That takes time, patience, and consistency. Most dogs improve significantly with the right approach — but “significantly better” might look different to different dogs. Some dogs manage to pass other dogs without a reaction. Others get to a place where they can be in the vicinity of other dogs calmly, even if close-up greetings aren’t their thing. Both are wins.

Progress also isn’t linear. There will be bad days, especially when your dog is tired, unwell, or has had a stressful week. That’s normal, not a sign that training has failed.

When to get professional help

If your dog’s reactivity is affecting your quality of life — if walks feel like a battle, if you’re avoiding outings, if you’re worried about safety — please reach out for professional support. Reactive dog training is complex, and having someone assess your individual dog, identify the specific triggers and emotional drivers, and build a plan tailored to your dog makes an enormous difference.

Private training is often the best starting point for reactive dogs, because it allows us to work at your dog’s pace in a controlled environment without the pressure of a group class. If you’re in Sydney’s Inner West or the surrounding area and you’re struggling with a reactive dog, I’d love to help. Find out more about private training here — let’s make walks enjoyable again.

Reactive dogs: understanding why your dog lunges and barks on leash