I've been revisiting some code I wrote a couple of months ago in order to implement some new ideas. It is amazing how much information you need to keep in your head to even start working on a modification of existing code. It took me a while to get used to the code again, thankfully I had written some simple comments.
At the point of writing the program, writing comments seems like a waste of time because all the information is already at your fingertips, but if you try to work on the program again after a while, you will find the comments invaluable.
There are other reasons why writing software is so difficult, see this Reddit comment.