As caring owners we make the best decisions we can for our dogs and unless we are with them from the day they are born and know absolutely the behaviour and genetics of their make up, we are working 'in the dark' to a great extent. And how many of us can possibly know all this information about a dog?
It's wonderful we achieve as much as we do with our dogs even if we know them from a puppy. We are asking a dog to trust that we will protect them in every circumstance occurring outside of their personal 'comfortably coping zone'; which differs and is influenced in range from a dog with a history of no quality (or hardly any) human contact through to the overly socialised. Ranging from the grossly abused dog through to the 'overly loved', from the neglected to the misunderstood.
This spectrum of dog behaviours is full of thoroughly differing personalities molded by what went before, and experiences of today.
If we start with a dog that is fearful and reacts to perceived threat, the dog is not, in his psyche, overreacting, as this is real to the dog and he is trying to keep safe. Rather like us if we panic.
In the throes of reacting, a dog's instinct is fight or flight, and any owner who can prevent this - in whatever way that works for their dog, including (as Tinyplanets has described) perhaps by distraction or perhaps by leaving a reactive situation - is behaving in the best possible way to protect their dog.