This has been said many times, yet many people tend to "forget" this rule. When you actually write your tests, the first thing you must do is reduce the risk of creating classes that are hard to test. You start with API usage and need to bend the implementation to best serve the API. This way, you usually end up with APIs that are both more pleasant to use and easier to test. When you're implementing test-driven development (TDD) or writing tests before code, you'll also end up implementing dependency injection, which means your classes can be more loosely coupled.
Doing this the other way around (writing your classes first and only then adding unit tests to them) may mean that you up with code that is easier to write but harder to test. And when testing gets harder, you may feel the temptation to skip it.
