I have heard this story of a biologist who studied howler monkeys. Every evening, as he returned to his cabin, one or more monkeys would gather to throw dung at him. One evening he was in a particularly bad mood and threw some of the monkey dung back. Before he knew it, every monkey in the community had gathered to throw dung at him. The biologist said the lesson of the story was not to get into a dung throwing contest with howler monkeys because they will never tire of it and you quickly will. I have found the idiom applicable to we human primates as well. When humans are happy we are peaceable and wise. When we are unhappy we can get addicted to petty squabbles. Returning tit for tat reduces us all to unhappy monkeys.