Human pain presents a number of obstacles to scientific researchers. These have been addressed previously.
Pain has been defined as a "sensation," and this in itself is an impediment to easy scientific research. Although the IASP definition is not the only possible definition to use, it does point out the distance between our current understanding of "sensation" and a full understanding. See Pain Definitions.
Pain is difficult to measure. It is known primarily through self-reports of ostensible pain sufferers. Since the severity of pain is of primary interest, this is a serious handicap to scientific understanding. See Pain Measurement.
It is extremely difficult to arrange pain experiments that approximate the natural conditions under which pain is experienced. See Pain Research.