One answer - Other People's Code

I know my code, I don't know your code. I don't know what you were thinking, why it's in the shape it's in. What assumptions you made that you might not have documented or who's documentation I have not found. Also, if you're the person who has to review it I have no idea what you'll accept which means I have to gamble wasting my time to make a PR on the off chance that you might not approve.

With my own code, all that goes away.

The other big one is yak shaving. Being forced to do things B, C, D, E, F before I can do the thing I wanted to do. I've lost projects to this where like say, some library has a bug, there's a fix in the new version, but the new version depends on newer things, and pulling those things in is not compatible with the project. I spent 30 to 60 - 120 minutes trying to get it to all work so I can get work, getting more and more pissed off, because this isn't what I planned to spend my time doing. And, I often fail and give up. Progress stopped on at least one personal project for ~4 years where once every 6 months or so I'd try to update dependencies and fail and never actually made any progress on the app.

Fortunately, this year, AI was able deal with it. It fixed the deps, removed most of them, and implemented most of the features on my TODO list from years ago.