Chances are you'll still be in no hurry to die if 100 years from now life is still rewarding. You may still feel the same in 1000 years. This is also an answer to those who might not want life extension. De Grey is not arguing for mandatory immortality. He's arguing against mandatory age-related death.
It's not life that gets old. It's the getting old that gets old.
People are funny creatures; too many of them think that they'll get bored with a life that is a few decades longer than the current standard. But I know from experience that I can get bored in a matter of a few minutes, an hour tops, if I really put my mind to it - as could anyone else. So if you can accept the possibility of an interesting and rewarding life of 80 years, why not one of 150? Or 1000?