There is no symmetry between (physical) pleasure and pain. There seems to be a distinct quality of pain (the pain quale) even though it can be experienced in different ways in different parts of the body. A toothache, a headache, a burn, a cramp etc., they can be sharp or dull, throbbing or stabbing, but it is as if the same essential pain is attacking you in different ways. It is associated with colours on the warm end of the spectrum, reds and yellows, diffuse or flashing, almost white perhaps. It is hard to think of a blue pain, although nausea, which is closely related to pain but not just a species of it, might be olive green. Pleasure, on the other hand seems to have no fixed quality, it is more a way of experiencing certain quite ordinary sensations, it might have no colour at all, but if it is associated with a colour that could be any colour at all. You can imagine a brown pleasure, think of chocolate or a good tawny port, or even a black pleasure, something like the memory of a deep sleep. Pleasure is mediated by positive states and often connected to certain neurotransmitters, like dopamine and serotonin, and other endorphins which suppress pain. Pain is thus treated as if it was a too direct coupling of the brain to external reality, while pleasure is mediation of any such coupling. Pleasure generally seems more diffuse, it spills over from its immediate site and causes every concurrent sensation to join in the feeling. Pain is single, coercing attention, while pleasure is symphonic. Sexual pleasure has the intensity of pain, feels as if it is unmediated and is often accompanied by pain-like groans, and yet is experienced as boundless pleasure without any distinct quality. No one can or seems to want to describe it in its aftermath. This might be just a piece of evolutionary trickery.