To understanding why the variable schedule is so powerful we have be capable of rationalizing it as humans. The variable reward schedule is best seen during gambling. Gambling addiction is among the hardest to overcome, despite having anything to do with outside substances. The inner chemistry does a good job of captivating us plenty. When the dopamine in our brain tells us that we have earned it than we keep doing whatever sets it off. Our dogs work the same way with their earning of reinforcement. Giving the treat is not the reinforcement in total, it is also the click and the right to the reward. It's like hitting the jackpot every time.