I have little qualms about living for centuries, particularly if my physical and mental health remain relatively high. I think it would be tremendously hard to bury my children and grandchildren, but grieving the loss your beloved doesn't compare to the time you spent together in life. If I cannot end my own life I'm not sure this is really a fair question that is worth discussing. Millions of years of joyful life don't compare to the trillions of years alone when humanity (and the universe?) is gone.
While people would love to live longer, it is a curse to live forever. Most people who think otherwise are afraid of death. I know this quote is from a movie, and movies aren't exactly an accurate way to argue over a topic, but in the X-men series, Wolverine is immortal. He has been evolved in many wars, like The American Civil War, World war I, War War II, and Vietnam. All VERY violent conflicts. Every night he has a dream about the wars hes been in, and the horrors he experienced. People normally are never involved with wars, but it is something to think about. In the TV show, The Twilight Zone, there is a man who was cursed with immortality and couldn't die. He was so old he met the Greek philosopher Plato. He begged people to kill him, although he was never successful. I believe these themes depict although living life is wonderful, we must legitimately live our lives and not prolong them, for it is a gift. If you are religious, you should embrace that it is the time to move on, or if you are a more spiritual human being.