Once again, you're making an assumption based off of your personal experiences. Having painful headaches a couple times a year doesn't compare to being in so much pain that you can't even function as a normal person. Everyone experiences pain in varying degrees, but few will ever approach the level of suffering these people go through. Even the toughest person on Earth has their breaking point, a time when they will say 'enough is enough.' If you couldn't even go to the bathroom on your own, which is downright humiliating, you might reconsider your stance the subject. Now imagine living like this day after day, for years on end. Many of these people love life just as much as any of us do. They've fought the good fight and just want to rest after a long struggle. Their families may even support their decision.
I appreciate your strong will to live, which is admirable, but try to see it from someone else's point of view. Death isn't such a bad thing, after all, it's just the beginning of another journey.