Why simple Zazen feels difficult

The form of meditation we do in Zen, unless we're working on a koan, is called shikantaza – nothing but sitting – or silent illumination. It's been called a "method of no method," in which we let go of any striving whatsoever – even to control our meditative experience. So can we do "bad zazen?" Theoretically, there's no such thing, and yet it sure feels like there is! What is this about?