Unless what you're testing has a very clear interface boundary (like a pure function whose behavior is unlikely changed, or the program's end-to-end behavior),

Unless what you're testing has a very clear interface boundary (like a pure function whose behavior is unlikely changed, or the program's end-to-end behavior), there's no clear distinguish between behavior and implementation. Most of the time you have classes that interact with one another. If you test each class individually, and you decide to refactor what classes do what, you'll need to rewrite a lot of tests, even though the higher level behavior should not change.