Most of us are wary of pain. We take all possible measure in every possible way to ensure that we avoid pain. And most of us carry a host of sprays and gels and pills to ensure that we stop pain, if it does manage to become a part of us. There is no way we can fathom and understand pain (like pleasure), yet we understand when people are in pain.
There are some, that experience pleasure in pain, whether inflicted on the self or others. The ability of pain to give pleasure is ridiculous to many. When you look at this concept in the context of certain philosophies of pleasure, it takes on different (potentially contradicting meanings).
I digress. (But it was relevant, in a way to the Pleasure Principle)
We feel most alive when we are happy, the entire world seems to belong to us. We want to dance, sing, talk, or do whatever it is that makes us express this full-of-life feeling. In pain too, we express, albeit differently. We cry, scream, shout or do whatever it is that makes us express the hurt. In a state of experiencing pleasure, there is a subconscious awareness that we are alive, yet there is no way to point to specific place or a thing and say, here, this is why; this is where the pleasure is. In pain, there is a heightened (I was thinking of using the word painful) awareness of being. It is local, it is specific, it is (almost) tangible. No other experience makes us as aware of our being as pain.
Pain is proof that we are alive.