Reactivity

Reactivity is a description of volume, not of motive. A reactive dog is responding far harder than the situation calls for, and the reason underneath it might be fear, frustration at not being able to reach something, pain, or simply never having learned that other dogs are unremarkable.

It matters that it is not a diagnosis, because the fix depends entirely on which of those is driving it. A frustrated dog that desperately wants to greet and a frightened dog that desperately wants distance look almost identical on the pavement and need close to opposite plans.

Lead reactivity is the most common version by far, and the lead is part of the reason: it removes the dog's ability to make distance for itself, which is the strategy every dog would otherwise pick first.