Going along with mindfulness requires a great deal of trust. Probably the beginning meditator will not be able simply to rest there, but will feel the need for a change. I remember someone who had just finished a meditation retreat telling me how she had sat and felt her body and felt grounded. But then she thought how she should be doing something else. She went on to tell me how the right book had "just jumped" into her lap, and she had started to read. At that point, one doesn't have a solid base anymore. One's mind is beginning to grow little wings. Mindfulness of body has to do with trying to remain human, rather than becoming an animal or fly or etheric being. It means just trying to remain a human being, an ordinary human being.
From "The Four Foundations of Mindfulness," in The Sanity We Are Born With: A Buddhist Approach to Psychology, page 27.