A failure to act gently jeopardizes the training program by undermining the quality of the dog/owner relationship.
Of course, aggressive behavior also runs contrary to the very foundations of most dog training techniques. Dog training is premised on the notion that positive reinforcement techniques best modify dog behavior. The interjection of inconsistent activity frustrates that process. Furthermore, there are ethical and moral concerns about mistreating an animal, in addition to the practical concerns. By making a commitment to retain gentleness throughout the training process one can insure they are consistently treating their dog appropriately.
A gentle dog owner will find himself with an ally for a trainee rather than a rival. Instead of a battle of the wills, training can become a cooperative exercise. Under those conditions, training tends to take less time and be far more effective.