Aside from stating the obvious—that punitive training methods can cause physical harm to dogs—let's look at some of the ways that positive reinforcement actually helps a dog's health.
As we often read about in health magazines or hear about in popular television shows on health, stress causes a plethora of health issues. These issues range from a compromised immune system, to joint stiffness and pain, to obesity and digestive problems, even to heart disease. This fact doesn't just apply to humans, but it applies to dogs as well. Stress doesn't just cause physiological problems, however, it also causes unpleasant psychological issues such as anxiety and depression as well—both of which further exasperate the above mentioned physiological effects.
I am a passionate animal lover, rescuer and trainer. Kindness is my goal. I never want an animal to feel intimidated or threatened. Training should be fun for both the dog and human, so the training methods I use reflect my goal of helping the animal to feel safe so they can learn and have fun. I desire the same for the human client as well. Life is too short to spend time training an animal in a fashion that is anything less than fun!