I find it helps when I consider the tests to be part of the code. If I need to change existing functionality, of course I'll need to change the tests so they test the new requirement(s). If I'm adding new functionality I add new tests to test the new requirement, but I shouldn't break any of the old tests when I do that. If I'm changing code with no requirements changes (as in, pure refactoring, not "tidying up as part of new feature development"), all the existing tests need to pass unchanged...