Quality of life is of the utmost importance to any thinking human being. Such people want to live at their most optimum level. If they are unable to do so, no matter what age, they would rather die than to not to live independently and/or on their own terms. The thought of depending upon others horrifies such people. They want to be independent as long as they can and when they cannot, they would rather die than to live a lesser life.
However, there are others who want to live a long life, regardless of the quality. They believe that they have the preordained right to live as long as possible. Even if their functioning is no longer at the optimum level, they still want to live. To them, life is life regardless.