The option of ending your life would eventually be far, far worse than the predetermined immortality.

Imagine it: you've lived for a million lifetimes longer than any other organism. You have traveled through the deeps of space, seen stars burst and fade, galaxies birthed and torn apart, life itself begin and end on a hundred planets a hundred different ways. And you ask yourself, "Is this all? Have I finally seen everything, experienced everything? Is there no more to teach, no more to learn, no more to feel and see and taste and touch and fear and love?" You have seen so much, you know the value of life, its fragility. Your own is not, but everything else is fleeting, every moment more precious than you can express. You have a rare gift. Even boredom is just another sign of your ability to feel wonder - you remember it, and you crave it anew. You ponder whether there are new wonders to be seen beyond death, but if there are, they are eternal. They will wait for you to decide. If you choose wrong, though, the material universe will spin out its unlikely beauty and horror and meaningless song without you, forever.

And here you sit, lost between the stars, a gun in your hand, wondering if you can bear to give it up, to surrender what so many have fought for, to end everything that you are when there are so many who are never given the choice. And you wonder if you've seen it all yet. And you know you never can, not til heat-death claims everything and you don't even have the energy to pull the trigger.