As programmers, we have long learned that Duplication is the Ultimate Sin of programming. Even considering to duplicate something is almost unthinkable.
But removing duplication introduces dependencies. If you and I use the reuse the same piece of code instead of duplicating it, changes I make may affect you. This effect can anything from beneficial (I fixed a bug you also needed fixing) to benign (I added a new feature that you’re not using) to detrimental (I want it to work in a way that’s no good for you).
When we have dependencies, we have to think: “Perhaps I shouldn’t add that feature – what if breaks something for someone else?” “Damn the torpedoes, I’m hacking it in!” or “Perhaps I’ll just make a fork for my changes and we’ll merge later”.