....do you have a goal, other than making $$$/retiring early/etc? ...
...do you think you will have accomplished something to help humanity?
...if you have kids, is your main goal to get them ready for life and mold them into useful, good citizens?
I've talked about this on this message board before. My dad died from a long, slow degenerative condition. The last few years of his life, he was in a nursing home and not able to speak, feed himself, or really move anything but one of his arms effectively.
At his funeral, when the pastor opened the floor for anyone to say something about Dad, a bunch of the nursing home employees who had attended - and nursing home employees don't go to funerals as a rule, because they'd never do anything else - stood one by one and told us essentially what one of them summed up the best: I would come in to work, and be having a really bad day because I fought with my wife, or someone cut me off in traffic, or I was worried about bills. And I would go to Harold's room, and as soon as he saw me, his face would just light up in this big smile. He was always so happy. He never got angry or mean, even when I was giving him treatments that must have hurt him. He took such joy in every little thing in his life. And I would tell myself, "If Harold can be happy in his condition, what right do I have to be unhappy in mine? He changed my life."
At that moment, I decided that if just one person came to my funeral and honestly said, "My life was better because she was in it," I would have had a good and successful life.