If you handed me acid I would feel pain. Moreover when acid interacts with something we can clearly see the erosive results as hissing, bubbling, slowly disintegrating. These are signs of chemical stress placed on the object. This obvious physical stress may well be experienced by the object as quantum "bings". I living organisms the resulting pain is a mental/neural experience of pain, an introspective warning signal that something is wrong. I don't think this is so remarkable. All plants experience "warmth" (infra-red), and "cold" (frozen). Even the paramecium experiences "touch", but that is not a mental experience, it is a local sensory experience (kinda like a roomba vacuum cleaner). Considering the astounding capabilities evolution has bestowed on various organisms, I don't see the experience of pain or pleasure as an extra-ordinary evolutionary feat. It's an efficient neural signaling system. As "memory" is an efficient long term mirroring system.