One of the challenges of rock climbing is that climbing up is much easier than climbing down. Usually, that isn't a huge problem. The goal of rock climbing is usually to end up on top, so you are typically not downclimbing. But sometimes you end up in a spot where you can't continue up and have to climb down. It is notoriously harder. Often hikers find this out the hard way. They encounter a rock wall and decide to try their hand at climbing - often without a rope. Then they get stranded when they realize they that climbing down is beyond their skill level.
What Does That Have To Do With Code?
Writing code is much easier than reading it. If you write code at your maximum level of cleverness, then by definition you are not smart enough to read and understand it. The trick is to make sure that you leave yourself some margin. Add to that the fact that code is written once and read many times and it makes a lot of sense to optimize for reading and not writing.