It's simple: They don't experience it. Before I got ill I couldn't comprehend it either. I only recognised really severe cases - like a young person who is about to die because of some horrible cancer. For a healthy person sickness and pain is temporary. Sure, you feel like hell for a week or two, you complain about the pain, but you know that it will get better soon.
The reason I'm here, the reason I want to CTB, is that with a chronic disease it never stops. I know that no medical treatment that is still possible will heal me. I know that it will only get worse, never better. And I know my suffering will only increase. The symptoms I experience now are manageable - but it could get worse in an instant and that also scares me.
In summary, "normal" people experience illness as something that will soon pass, they hope that it will be over and they will be right. They can't comprehend a disease that will literally never stop destroying their body and the pure hopelessness that results from that knowledge.