Kent Beck, XP Explained:
It is impossible to test absolutely everything, without the tests being as complicated and error-prone as the code. It is suicide to test nothing (in this sense of isolated, automatic tests). So, of all the things you can imagine testing, what should you test?
You should test things that might break. If code is so simple that it can't possibly break, and you measure that the code in question doesn't actually break in practice, then you shouldn't write a test for it...
Testing is a bet. The bet pays off when your expectations are violated [when a test that you expect to pass fails, or when a test that you expect to fail passes]... So, if you could, you would only write those tests that pay off. Since you can't know which tests would pay off (if you did, then you would already know and you wouldn't be learning anything), you write tests that might pay off. As you test, you reflect on which kinds of tests tend to pay off and which don't, and you write more of the ones that do pay off, and fewer of the ones that don't.