Everything in moderation. Even moderation.
Essentially, the key to a happy life is not to over indulge, but even a lack of overindulgence must not be overindulged all the time. My mother has given me this advice on multiple occasions. I’m pretty sure she made it up, but I have told it to many people since I believe it to be good advice. Primarily when I want to eat a large amount of unhealthy food.
The proverb itself is guarding against unhealthy behavior and provides insight onto the human psyche. No one can be in control all the time. When you try to moderate everything you do, your desires become uncontainable and will find a way of expressing themselves, often in unhealthy ways. By making sure to moderate your moderation you are dealing with the issue of keeping your id at bay.