Five hours ago I was reviewing some failed tests in a PR. The affected code was probably 300 lines, total source for the project ~1200 lines. Reading the code, I couldn't figure out what the hell was going on... and I wrote all the code. Why would that be failing? This all looks totally fine. <changes some lines> There that should fix it! <runs test suite; 6 new broken tests> Fuck.

When you write code, your brain follows a logical series of steps to produce the code, based on a context you pre-loaded in your brain in order to be capable of writing it that way. The reader does not have that context pre-loaded in their brain; they have to reverse-engineer the context in order to understand the code, and that can be time-consuming, laborious, and (as in my case) erroneous.