The tenth slice of pizza never feels as good as the first
Each additional unit of something you consume tends to deliver less added satisfaction than the one before it — the first cold drink on a hot day is transformative, the fifth barely registers. Economists used this to explain why demand curves slope downward and why a fixed sum of money matters more to a poor person than a rich one, since each extra pound buys progressively less added wellbeing. It's a foundational building block of modern consumer theory.