Because of science, of course!

 

It has been proven that using aversive training tools/methods (shock/prong collars, physically hitting) can cause both physical and mental damage to dogs over a period of time.

Fear/Force-free trainers take into consideration both physical and mental well-being.

 

Here are some articles on why aversive methods do more harm than good:​

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The Implications of Punishment

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Humane Dog Training Position Statement