HOW DOES LEARNING HAPPEN ?
Dogs ( and other animals ) learn through a process of association. ABC - Antecedent (what starts the behaviour ) Behaviour , Consequence .
If your dog is emotional ( over threshold ), your dog will learn that the antecedent predicts the consequence . Eg if your dog sees and reacts to a dog and is punished for it , it will not learn that it's behaviour results in punishment , it will learn the ANTECEDENT - seeing a dog - predicts punishment . This will result in increased reactivity when it next sees a dog and expects to be punished again .
In order for your dog to learn that BEHAVIOUR predicts a consequence it must be calm .In order to do the wanted behaviour it must be rewarding. There are various techniques to teach this while keeping the dog calm so learning happens. Eg the dog learns the antecedent predicts a REWARD. Therefore reactivity lessens because the dog forms positive associations with the antecedent and learns to focus back to owner for reward, and with practice stops reacting.
Psychology . The reason why qualified positive reinforcement training should be your only choice .