Not too easy; not too hard. Just right. First time I've heard of this concept. Although when I apply it to playing basketball, I like to find the sweet spot where I'm just slightly better than the competition. From Lifehacker.
How to Stay Motivated in Life and Work Using the Goldilocks Rule
Human beings love challenges, but only if they are within the optimal zone of difficulty.
For example, imagine you are playing tennis. If you try to play a serious match against a four-year-old, you will quickly become bored. The match is too easy. On the opposite end of the spectrum, if you try to play a serious match against a professional tennis player like Roger Federer or Serena Williams, you will find yourself demotivated for a different reason. The match is too difficult.
Compare these experiences to playing tennis against someone who is your equal. As the game progresses, you win a few points and you lose a few points. You have a chance of winning the match, but only if you really try. Your focus narrows, distractions fade away, and you find yourself fully invested in the task at hand. The challenge you are facing is “just manageable.” Victory is not guaranteed, but it is possible. Tasks like these, science has found, are the most likely to keep us motivated in the long term.
Tasks that are significantly below your current abilities are boring. Tasks that are significantly beyond your current abilities are discouraging. But tasks that are right on the border of success and failure are incredibly motivating to our human brains. We want nothing more than to master a skill just beyond our current horizon.
We can call this phenomenon The Goldilocks Rule. The Goldilocks Rule states that humans experience peak motivation when working on tasks that are right on the edge of their current abilities. Not too hard. Not too easy. Just right.