Two children share one cake and one child's gain is the other's loss. Two neighbours swap mangoes for apples and both end up better off, with no new fruit. Telling these apart changes what you should do.
Holding out two weeks for a better house price is worth a fortune to you and pocket change to your agent. No dishonesty required — just a small gap between what you want and what they're paid for.
Every herder who adds a cow to the shared pasture captures the whole benefit and pays a twentieth of the cost. Nobody is greedy, everyone is correct, and the pasture dies anyway.
Species that have survived for ten million years go extinct at about the same rate as new ones. That surprising fact explains why your improvements so often buy you nothing.
Betraying your partner is the correct choice no matter what they do. So both of you betray, and both of you serve five years instead of one. No error was made — and that is exactly the problem.
Two petrol stations cut prices until neither can move without losing. Nobody wanted the result, everyone behaved sensibly, and it held anyway — because stable and good are different properties.
A rising power wants influence matching its strength; an established one sees each concession as erosion. Both are sincere, neither needs to want conflict, and the pressure builds anyway.