Boredom? When I'm debugging someone else's spaghetti code it's not interesting or rewarding. It's mostly tedious bookkeeping that shouldn't even be necessary. It's the difference between compiling a custom linux kernel for yourself and removing malware for a friend. The former is intellectually engaging, rewarding, and fosters a sense of accomplishment in making something work. The latter is a tedious abomination that should have never happened.

The only times I enjoy spaghetti code is when I have to learn a tool or more about a language or library to debug or understand it. At least then I'm engaged and learning what to avoid and how to fix it.