Why is my dog only reactive on the lead?

The short answer!

The lead removes your dog's escape option, and when they're unable to run away from something scary, they bark and lunge to make it leave instead.

Leash reactivity happens because the lead removes your dog's escape option.

Off-lead, a nervous dog can choose to run away if they need to, creating distance from whatever worries them. On-lead, that choice disappears.
When escape is blocked, the sympathetic nervous system shifts into defensive mode. Your dog can't retreat, so they escalate to threatening behaviours: barking, lunging, hackles up, stiff posture.
The goal isn't harm, it's creating space by intimidating the trigger into moving away.

Why it becomes self-reinforcing

The behaviour works! The other dog keeps walking, the jogger runs past, the cyclist rolls on by - your dog's brain registers: 'I barked and lunged, the scary thing left, I'm safe. This reinforcement loop is why leash reactivity often worsens over time without intervention (1).

What you might notice

Early signs: increased lead tension when another dog appears, brief staring, slight body stiffening
These escalate quickly to explosive reactions if the trigger gets closer
Threshold distances vary: fine at 20 metres, exploding at 10 metres
Behaviour is context-specific, perfectly sociable off-lead at the park but reactive on pavement walks

Individual factors

Barrier frustration behind fences or windows follows similar logic, the physical restriction amplifies the emotional response. You might have seen videos online of dogs being aggressive from behind a gate or a fence, only to be very 'happy go-lucky' immediately after its been removed. Dogs bred for vigilance or high arousal thresholds (terriers, herding breeds) may develop leash reactivity more readily. Rescue dogs with unknown socialisation histories often show stronger patterns because they've learned that this sort of behaviour works in unpredictable environments.

References

Herron ME, Shofer FS, Reisner IR. Survey of the use and outcome of confrontational and non-confrontational training methods in client-owned dogs showing undesired behaviours. Appl Anim Behav Sci. 2009;117(1-2):47-54. Available from: https://www.sciencedirect.com/science/article/pii/S0168159108003717

This content is for general information only and does not replace veterinary advice. Supplements and nutritional products are not licensed veterinary medicines. Always speak to your vet before making changes to your dog’s diet, supplements, or care routine.

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owners also ask

What‘s the difference between reactivity and aggression in dogs?

Reactivity is emotional flooding, aggression is intentional harm. Reactive dogs are overwhelmed and trying to create distance, aggressive dogs are determined to cause damage.

How to calm a reactive dog

In the moment, create distance from the trigger immediately. In the long-term, structured desensitisation and counter-conditioning work are most effective.