We've all seen rivalries: Yankees vs. Red Sox, Coke vs. Pepsi, Dawson vs. Pacey. So what's all the hating about? This isn't Star Wars with its distinct light and dark sides. No. We may prefer the Red Sox over the Yankees (and you'd better, too), but we can't really say the other side is evil.
The Yook-Zook rivalry follows the same rules, but somehow the rivalry turns into an all-out arms race. Why? Not because of what side of the Wall they live, but because of a cultural difference: these two groups butter their bread differently. The difference is substantial enough that we can clearly say that Yooks are not Zooks and Zooks are not Yooks. And this difference gives each group their own sense of pride, even if it's based on how a Yook and a Zook way back in ancient history decided to butter their bread.
We can all relate, right? After all, we're all part of our own little groups. Sure, we don't have arms races between nerds and jocks, but Seuss makes a strong point about the animosity that can form from excessive pride in one's group. According to Seusspert Ruth K. MacDonald "the author's point is that there is little difference between the two sides in the arms race" (source).
And Seuss backs her up: "'what I was trying to say was that the Yooks and the Zooks were intrinsically the same'" (source). The buttering differences might seem big to us when we focus on them alone, but if we dig deep, we might just find that we're not so different. Cheesy? Yes. Accurate? We sure hope so.