For some dogs, walking on a lead is no different to walking without, but for others, the restriction of movement and limitation of their options to move closer to or indeed away from something else can be problematic.
For some dogs, being on lead removes the "flight" option, ie. the ability to move away from something they don't want to be near, and leaves them only with the "fight" option. Having them on a tight lead can cause their fear to manifest in protective and defensive behaviour, as they aren't able to exercise another option.
On the other hand, and in the other direction, being on lead can also cause serious frustration, as they can't reach the other dog, thing, place or person they want to pull and pull towards.
Negative experiences while on lead can also create a progressive cycle of negative emotions leading up to the event, to the point where even the simple appearance of the lead at home before even stepping out of the house already sets off the behaviour.
Similarly, signals coming from the owner can reverberate in the dog, as they begin to anticipate an ominous "something" happening from their parent's cues. As the parent begins to prepare for, worry about or even dread the behaviour that might manifest when their dog meets someone or something when on lead, subtle changes in their own human behaviour may in fact be feeding their dog's behaviour. This can be things like tightening up on the lead, a change in voice, tone, body language, breathing, scent and even reaching towards the treat stash all in preparation of what's to come!
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