The most frustrating thing for me is probably that people think because you look normal then you have no health issues. It has really made me re-think how I look at other people. From people in wheel chairs, people who drive with blue badges, people who limp, people who are blind, or deaf; I really feel like going up to them all and saying it is OK you are normal like the rest of us ... I cannot sleep or breath. I do not see why we measure an ability as if one is more important than another.
I do not like other people's reactions to me for example when they frown and say, "Oh, he's asleep again" as if I do not get enough sleep at night. Incidentally I do not, but that is not the reason I fall asleep during the day, it is because I suffer from Narcolepsy; I do not explain that to others, they do not understand and it would get into a long discussion about what Narcolepsy is and why I can still function even with the condition.
You can never know everyone's life story. For example, the man with no legs may be a war hero from Afghanistan or Iraq, or he may have just come home too drunk one night and driven himself into an accident. I am not to judge, but I can still give him the same thoughtfulness for his condition.
So in summary my frustration is probably the lack of concern or understanding you get from others. You would hope your family and friends would at least be a little more considerate.