Why do most people fall on their face at some point, before reaching their goal? It's all or nothing isn't it? 100% or bust. Couch to 5k. Working out 5 days per week and never skipping, 3 or 4 well balanced meals to hit every angle from protein uptake to micronutrient ratios. It's something that we're accustomed to doing. Always going in whole-heartedly.
And that's totally fine! Where it's wrong is thinking perfection, and 100% effort are the same thing. This is where the 80/20 rule comes into play. We can never ever be on all of the time, 24 hours per day, every day of every week. That's just not possible.
True balance in fitness and health is 80% the good stuff, you know what I mean? Eating this, and not that. Choosing exercise over a movie. Playing with your kids instead of being a couch potato. But the 20% is a slider accounted for error.
Error being the falling on the face. You will, it's just a matter of time. You'll eat that donut. You will wake up some mornings with the “nope” immediately in your head. Your human. It will happen.
So we calculate that into our programming. No one can eat perfect all the time. No one feels like working out 7 days per week. And no one does it all perfectly, all of the time. There are strides and there are plateaus, but the 80-20 rule can keep sanity and persistence in place long enough to hit any goal you desire.