Why? As I said, I'm a believer in quality of life. Sure, we can keep people alive, and delay the inevitable, but at what cost? I'm not talking just financial (although that is HUGE), but emotional and physical.
In my case, I have an incurable (but fortunately slow growing) cancer. I've had surgery that removed as much of it as possible (18" of gut, 5 tumors, mets on the bladder and peritoneum, plus all the lypmh nodes that could be removed). There are two tumors that cannot be removed. Typical progression is that it will eventually spread to my liver, at which time I'll have additional procedures to remove (or chemically destroy) the affected areas of the liver. There's about a 40% chance that it will get to my heart, and I'll need to have valve replacement surgery to treat the right sided heart failure. There's about a 30% chance that it will spread to my bones, which is incredibly painful. Eventually, I'll run out of liver. Liver failure is a fairly slow, and VERY miserable way to die.
Lying in a bed, stoned on narcotics, unable to care for myself, is not living as far as I am concerned. I will not do it. I will not subject my family to it.
At this point, other than knowing I've got limited time and such fun things as periodic bouts of diarrhea, the cancer doesn't really affect my life. I continue to work, continue to train, and continue to do the things I enjoy. My kids are raised. My wife and I are spending my retirement savings now, traveling to dive destinations and seeing the places we planned to see after retirement. We're enjoying life.
When it's no longer possible to be productive and enjoy life, then it's done. Dragging out the inevitable while draining my families resources serves no purpose. So I'll check out, with a minimum of fuss and pain.