Threshold is a word you don't hear much, but it is such a crucial key to understanding dog behavior and helping your dog develop into a balanced, happy dog. We humans (aka primates) relate to dogs as if they were humans too. We stare at them, trying to establish eye contact (a threat in canine body language), we hug them (do you see dogs hugging?) and pat them on the head. We want to socialize them so we take them out and throw them into uncomfortable situations - dog parks, farmer's markets, family parties! Understanding threshold, and learning how to keep your dog under threshold, and what to do if they go over threshold are important skills. Please read this article. 

http://www.whole-dog-journal.com/issues/16_4/features/across-a-threshold_20726-1.html?zkPrintable=true