Pain is a very subjective sensation - people don't all feel pain the same way. We also have a tendency to want to be able to see what is causing the pain. How many times have you heard someone looking at a bad wound say, "oh, that's got to hurt"? If people don't see the injury happen or see the results, it's often not something that they consider as being very painful.
Another reason why it's difficult to get some people to understand the chronic pain is that many people who do have chronic pain have good days and bad days. On good days, they can accomplish a lot, on bad days, not so much. If people see them running around on good days, they may question how bad the bad days actually are.
The only way you can help people understand is through education. If you can help people understand the problem that is causing your pain, they may be more understanding of the pain itself.