Before moving on to the third of the four foundations of mindfulness, let's look at one more aspect of the second, mindfulness of feelings. We've been talking about being mindful of emotions, but of course "feelings" include physical sensations as well.
Reading about Buddhism, one could get the impression that a practitioner must renounce the enjoyment of "worldly" sensations. But this is not right. Mindfulness of sensations is a direct awareness of sensations without attachment to them. In other words, it's not "my" pleasure or pain; it's just pleasure or pain.
The late Chogyam Trungpa Rinpoche called the second foundation "mindfulness of life." He pointed out that pleasant and unpleasant sensations are tired to the survival instinct. A sense of "pleasure" evolved so that living organisms would engage in things that would help them survive and procreate, while pain and discomfort are warning signs that something is wrong.
We clever humans, of course, have managed to concoct many things that please us but which we'd be better off without. But at a very basic survival level, living organisms are bound to their sensations and must attend to them, or die. Pleasure and pain cannot be avoided.
Trungpa described working with the second foundation as "touch and go." He said, "You focus your attention on the object of awareness, but then, in the same moment, you disown that awareness and go on." It is awareness without attachment or avoidance.
This may seem a subtle point, but it really is important to understand the difference between experience and attachment. There's nothing wrong with enjoying a pleasant sensation. However, we get into trouble when we attach to the experience as "mine." This sets of a cycle of regret when the sensation passes and a craving to bring it back again.
We do the same thing with pain. It's said that "Pain is inevitable. Suffering is optional." Put another way, pain is a physical sensation; suffering is how we choose to experience it. (Full disclosure -- I'm a terrible wuss about pain and cannot say I've gotten the hang of the pain-without-suffering thing myself.)
Sometimes it helps to remember these lines written by the scholar Buddhaghosa (5th century CE):
Mere suffering exists, no sufferer is found.
The deeds are, but no doer of the deeds is there,
Nirvana is, but not the man that enters it.
The path is, but no traveler on it is seen.