As painlessly as possible, having suffered suddenly from Ulcerative colitis, and having endured for the best part of a month, a white, blinding, screaming wall of pain whilst I wasted away so that I was days from death as my bowels gave up, started to come apart and then finally rupture, I sure as heck don't want to have to go through that again!
I have never felt pain like it, and I don't ever want to feel it ever again either!! (It wouldn't have been so bad, but it was the fact that it was over such a long time and there was sod all I could do about it). The upshot is that after staring death in the face, I am now not afraid of dying.
Also, I don't ever want some disease that affects my wellbeing, be it something like heart problems or worse, some sort of horrible degenerative condition, I want to be fit right up until the end!!
I carry a donor card and my family know of my wishes and accept it (I am also registered), so even if I do go early, I will still be able to help others!