Fantasy has vampires prolonging their life by drinking blood, while science fiction offers medical immortality or the uplifting of consciousness to machines. Readers, quite obviously, like to play with the idea of living forever. Yet the more I think about the possibility, the less I’m convinced that everyone is suited to immortality.
Of course, if immortality (and a reasonable degree of youth) ever becomes a possibility, I imagine that it will be reserved for the rich. I imagine, too, that if immortals are in charge of the process that created them, the selection process will be rigorous. There’s an issue of Garth Ennis’ graphical novel Preacher in which the vampire Cassidy meets the first other vampire in over a century. To Cassidy’s disappointment, his fellow vampire turns out to be “a bit of a wanker. ” After trying to reform him, Cassidy ends up leaving him to be destroyed by the rising sun – an ending that is only sensible if you stop to think about it. A long life is going to lose much of its zest if you have to spend it with highly annoying people, so immortals would want to choose their peers with care.
However, what I am really thinking of is that most people are simply not equipped for a long life. For the kind of person for whom time is something to fill – the kind who spends their weekend shopping, or their evenings at a bar – living seventy years or so can must be hellish enough. Several centuries of filling time would probably end with such people committing suicide, or perhaps turning violent in the hopes of finding another thrill.
Either seems a waste of immortality and a source of unhappiness for everyone. I suspect that good candidates for immortality would be those who know how to keep busy, and never have enough time for everything – artists and artisans of all sorts, and scholars.
Yet even being inner-directed might not be enough for immortals. Many artists do not have seventy years of work in them, much less several thousand.
It strikes me that a well-adjusted immortal would have to maintain a fine balance. On the one hand, they would need a strong tolerance for routine. The English playwright Christopher Fry remarked that the problem with being 94 was that time seemed to move so fast that he seemed to be eating breakfast every five minutes, and the problem would probably only get worse with the centuries. A well-adjusted immortal would have to be able to endure all the repetitive eating, urination, sleeping, grooming and sex without boredom setting in. Better yet, routine would need to be a comfort for them.
Yet, on the other hand, a suitable immortal would also need to accept change without falling into the traps of condemning or embracing everything new or living in nostalgia and gradually falling hopelessly out of touch. This is a balance that ordinary mortals struggle with, but I imagine that successful immortals would be those who could live the same routine for twenty or thirty years, then shake it off and find a new routine to settle into.
However, perhaps all this is beside the point. Perhaps the human brain and/or consciousness simply isn’t equipped for a longer life span. Perhaps a point arrives for everyone at 70, 90, or 110 when the sense of self simply collapses into senility, overloaded with memories and perceptions. But if humans ever manage to live significantly longer, those who manage to do so with any degree of contentment will be only a very small percentage of the population. For many immortals, the mental torment might make them think Tithonus had things good, living forever but aging into a cricket.
Off the Wall 
Well clearly there’s no such thing as living forever literally (not even the known universe will truly last forever). When we say “immortal”, we’re really talking about “much longer than people do now” or even “as long as they choose to live”.
Also, there’s more than one type of immortality. Being able to greatly extend the lifespan of the physical body (but still without thoroughly understanding it) is quite different from being able to “back up” your consciousness to a machine storage system, for example. And of course, there are questions around whether you are simply archiving the information in passive storage, or immediately reincarnating it.
I think the latter has the more interesting consequences — a daring individual might approach dangerous situations by “saving often”. In Tanith Lee’s “Don’t Bite the Sun” one of the characters is a serial-suicide, because he craves the ‘reality’ of death in their highly-artificial environment. They’re all basically immortal teenagers who never fully mature (well, the story is about a form of maturation).
We also haven’t really fully explored the possibilities of consciousness merging or splitting. It may be that some day these will be options, allowing personalities to be reintegrated in new ways, thus preserving some of the renewal benefit of recombinant reproduction, but without losing so much of our life experience from one generation to the next.
I think that death “as we understand the term” may have little or no meaning at some point in the future — there being a lot more possible states than just “alive” or “dead”.
I also think there’s a certain irrationality involved in this discussion that has to be acknowledged consciously. We’re naturally afraid of death, and people are at different stages in dealing with the idea of mortality. Much of the discussion of longevity or immortality comes not from logical reasoning but from trying to rationalize that fear.