The Unit Test Paradox
The unit test paradox is the paradox that low level unit testing "drives" good design by itself being a form of bad design.
Low level unit tests tightly couple to code
All tests couple to something - end to end tests couple to the body of the architecture as a whole. Lower level integration tests couple to lower level implementation details.
The lower the level of the testing, the tighter it couples to the implementation.
Coupling == pain
If you have worked on tightly coupled software or "spaghetti code" you will understand the pain of trying to understand