I think some dogs are just wired different. I had a Rot that was the most stuburn, irritable dog I have ever dealt with. He would growl when I pet him, Show his fangs when I clipped his nails, washed him, or brushed his coat. As hard as I tried, I was never able to fix this. Luckily he was all show and no go. I kept that dog 13 1/2 years, and never once did he ever cause anyone problems.
I almost think it was a game for him, As his owner He never once scarred me. Honestly, I miss that old stuburn fool. That dog truly challenged all my training abitlities, by the time he was 6, he was near perfect, except when cliping his nails, giving him baths, or brushing his coat.
I think he was lucky to get me, and I was also lucky to have him. He probably wouldn't have lasted in every situation.