Even with a DNR or living will, that does not allow one the choice to say "enough".  I had a grandfather who lost his larynx and esophagus to cancer.  Near the end, he was draining out whatever food was injected into his feeding tube so he would die sooner.  Could not talk, could not breath normally, and could not eat normally.  Please, show me anyone in that condition who would want to live another year.  Thankfully, he did not live much longer than a year.