The following is an excerpt from our upcoming book on dog training..


STRESS and PLEASURE in DOG TRAINING
I will start with stress as this is a topic often misunderstood in dog training. Stress is something many trainers and owners fear and avoid at all costs often to the detriment of the results they produce and ultimately the dog they are responsible for helping. That is because they refuse to think about stress logically and are never able to get past their emotional response to stress.  Stress is neither good or evil, it just is. Stress is simply a mechanism that all living beings experience to help us process the world around us. Namely what to avoid.
If you think about it from scientific standpoint stress is simply a mental or physical state that is uncomfortable. Stress can be major or minor and it is a biological imperative that nature provides all living organisms to ensures their continued survival. In the natural world stress creates behavior that is conducive to continued success as a species and survival as an individual.
Example: If the dog is hungry that uncomfortable hollow sensation he feels in his stomach that pushes him to search for food to ensure he has the calories necessary to survive is stress.
Example: If a young feral dog in a pack offers behavior that is undesirable to the social structure of the pack and is punished by another member of said pack. The stress derived from that physical confrontation can lead to the dog avoiding the undesirable behavior in the future. This makes it more likely that he will be able to remain within the pack and not suffer injury or be driven away from the group for anti social behaviors.
Now I want you to think about what we want out of our dogs, whether its functional obedience or simply for the dog to avoid certain dangerous behaviors.  Many of these things do not come naturally to the dog even with a thousand or so years of domestication. Things like running into the road or chewing your $5000 couch may be completely natural behaviors to your dog.  If you think you can reliably teach a dog to avoid certain behaviors simply because you make an alternate behavior pleasurable (the go to method for positive only trainers) you do not understand how extinction works and more importantly have not successfully trained enough dogs.
Finally, I want you to think about the human world from our Education Systems to our Criminal Justice system. Stress is an inherent part of how these institutions operate. If you do not do well in school you will suffer from numerous social and financial penalties. If you break the law you may suffer fiscally or lose your freedom.  Even we humans who are capable of complex verbal communications and intellectual reason require stress to be successful on the micro, macro and mezzo level. Truly it is the height of ignorance and selfishness to hold dogs, creatures that function on an instinctual level to standard that we as humans cannot even exist at.   
As trainers our goal should be to create a dog that is reliable in the obedience and free of dangerous behavior. Dangerous behaviors like running away or aggression lead to more dogs getting killed or being surrendered then anything else.  Anything less then this for some utopian ideal is unethical to the dog and owner that pays you to help them. Stress to some degree is a necessary part of this process, see the section on Operant Conditioning if you are still confused.
Now pleasure is just as powerful as stress but in a different way. Pleasure is not something I need to write much on because most people understand why it works and why it is good in dog training and in life. When you are able to pair desirable behavior with pleasure you make it more likely to reoccur and also pair a positive emotion with said behavior. If you want to speak about ethics, all trainers should use pleasure as much as humanly possible to promote the good but never at the expense of allowing the bad.
Pleasure and stress are all biological imperatives that exist to teach us about the world around us. They are especially important to creatures like the dog who cannot speak or be reasoned with.  Using these sensations in a strategic way to promote desirable behavior and eliminate undesirable behavior are prerequisites to a complete obedience training system.