Where It Comes From

Reactivity is rarely a character flaw. It usually traces back to one of two roots.

• Missed socialization. Puppies have a critical window, roughly three to fourteen weeks, when the world is supposed to feel safe and normal. Dogs who miss it often find ordinary things alarming for life.
• A bad experience. One frightening encounter — an off-leash dog charging, say — can stamp in a lasting fear.

It also lives on a spectrum. Some dogs react to everything; others only to a specific trigger, like big dogs or men in hats.