I see your point and it's interesting to think why that is so. When I test the implementation I am basically writing tests for myself to help convince myself t

I see your point and it's interesting to think why that is so. When I test the implementation I am basically writing tests for myself to help convince myself that the code is doing what I think it does. Whereas testing the public interface helps other people in the same manner. Note it is possible that a test of a private method might fail yet all public methods calling it would still work correctly according to their "spec".