It's not rocket science by any stretch. It is a simple concept. In many situations, after a certain point increasing the magnitude or intensity of a particular factor results in a negative decrease in the overall effect. But before the point, the increase corresponds to a positive increase in overall effect.
For example:
Increasing the confidence of someone with low self-confidence creates a positive effect.
Increasing the confidence to someone with lots of existing self-confidence creates the negative effect of over confidence.
Take in Too Little calories and you are skinny. Take in Too Much and you get fat. There is a Just-Right amount of caloric intake that helps you maintain your ideal weight.
There are so many examples of the Goldilocks Law that it is helpful to think of an example of what doesn't fall under the Law.
Judgment and decision making skill and ability, you really can't get too much of that.
Food for thought:
How many arguments and disagreements are the result of people viewing the situation from different sides of the Inverted U-Curve. In other words, people with seemingly opposing viewpoints may actually agree on the fundamental concept. But they may disagree on whether a certain (known, perceived, imagined) quantity of that concept is actually positive or negative.