Believe me, even when the symptoms are very visible or measurable by labs, most people around you will still not get it except perhaps if they suffer from a very similar condition.
By our nature each of us is obsessed with his/herself and don't want to be bothered by the problems of others, even at the same time we are wondering why other don't care about our problems. We simply think that "I" have enough problems and cannot add the burden of others to them. We justify this to ourselves in hundreds of ways in order to pacify our conscious and not add another reason to lose valuable sleep.
Furthermore, while it is generally hard to communicate and share any issue with others, this becomes even more difficult when the parties involved do not share the same experience(s).
If you have never been to the circus you would not understand references to what happens there. Similarly, if you were never handicaped you will not be able to understand the life of one. There are also millions of types of handicap and each is absolutely unique a difficult to grasp. This becomes even more difficult to relate to when one is trying to communicate to others a feeling, like pain. The best that medical geniuses were able to come up with is asking you to give your pain a value on a scale of 1 to 10. What? My pain this time is 7.35. No, I think it changed. It is now 7.7, but it temporarily goes up to 8.66 or drop to 5. Or since I was just been shot the pain felt like 10 but I discovered years later that passing a kidney stone was even worse. Should I rate passing the stone as a 14 or should I go back to my medical records and edit all my pain scales?
I know that this silly but I am trying to make a point about how difficult it is to really express pain in a way they could understand. What one cannot understand would usually be discounted and brushed aside.