An interesting phrase, no? The research summarized here says that dogs do not have an infinite supply of self-control. They reach a point where they can no longer exercise the self-control needed to stay out of trouble. Or, as Julie once said to me after I described Chico being good all day and then unreasonable, “Yup, he’d reached the end of his good nature.”
And in the big wide world, it’s our job to observe, and learn, and judge when they are just about to have had enough. Not easy work.