For life—the life of any sentient creature—to be worth living, there must, as Robert Ettinger has often said, be a preponderance of satisfaction over dissatisfaction. If this overall slant toward good rather than bad is maintained, it seems reasonable that one stands to gain by continued existence. I am not sure what fraction of the human (or other sentient) population achieves this positive balance and will not speculate except to note that by appearances there are many humans who do achieve it, along with other creatures, pets in particular, so at least for them, life is worth continuing. To say that life once started is worth continuing does not, as David Benatar points out, imply that it was worth starting in the first place, or should have been started. But I think that, barring certain problematic cases, it is fair to conclude that a human life at least is worth starting, if there are responsible prospective parents who would like to start it. Here I think it is reasonable to expect that the resulting person will feel that life is overall a benefit, and additionally, that others, the parents in particular, will stand to gain from the new life that has entered their lives. I don’t accept Benatar’s arguments that by and large life is pretty terrible and people delude themselves who think otherwise.
Also I reject his “asymmetry” argument, that it is “good” if a life that would be bad does not come into existence, but merely “not good” rather than “bad” if a life that would be good does not come into existence. (It is easy to see how this asymmetry supports the argument that life should not start in the first place and Benatar refers to it often.) Benatar’s main rationale for this argument seems to be that, while we would consider someone morally at fault for deliberately bringing into existence someone who would be miserable and just want to die, we would not similarly hold someone culpable who elected not to bring into existence someone who would be happy and want to remain alive. This I think should not be the only consideration, for it is based only on the idea of when we should regard an action as bad, and not at all on when we should regard it as good and commendable. (Why this particular asymmetry?) Instead, weighing both sides of the issue as I think is justified, I would opt for the fully symmetric position that it is “not bad” if a life that would be bad does not come into existence, and similarly, “not good” if a life that would be good does not come into existence. On the other hand, I question and doubt whether a life that comes into existence would be bad in the long run, given the prospect of immortality, which I think is a possibility through science (see below).
Life does, of course, have its problems, death in particular, that might call in question whether it is worthwhile after all and thus, whether the life of any sentient being is worth starting. For this one problem there are a number of possible answers that will be satisfying to different people, and thus can serve as ground for a feeling that life is worthwhile and was worth starting despite one’s own mortality. There is the famous Epicurean argument that death is not really a problem because before it happens it causes no harm, and after it happens there is no victim. There is the Buddhist argument that, more fundamentally, the self is an illusion anyway, so that in fact no persons exist and death never really happens, though bliss can still occur through states of enlightenment which thus are worth seeking. There are various religious traditions that promise an afterlife and a happy immortality for those who prove worthy, or, in some versions, all who are born. Then there is scientific immortalism, which holds that at least substantial life extension through science and technology is possible, so that, irrespective of any supernatural or mystical process, persons of today have more to hope for as they get older than the usual biological ruin and oblivion.
The scientific possibilities for overcoming death come in different varieties that each have their own advocates. Some of these hopefuls, particularly younger ones, focus on the prospect that aging and now-terminal illnesses will be remedied in their natural lifetime, so that they will escape clinical death and need not specially prepare for it. Others who are not so confident have made arrangements for cryopreservation after clinical death, in hopes of resuscitation and cure of aging and diseases when the requisite technology becomes available. Still others hold out for advances on a more cosmic scale that will eventually make it possible to raise the dead comprehensively. (Some possible scenarios for this using multiple, parallel time streams rather than revisiting or recovering a hidden past are considered in my book, Forever for All, and the article at http://www.universalimmortalism.org/resurrection.htm.) The three possibilities are not mutually exclusive, so that, for example, persons who have chosen cryonics may also place varying hopes in the other two. In fact, my personal viewpoint as a scientific immortalist grants some validity to all three possibilities, but I think it is imperative now to be engaged in cryonics, which is almost unique and the clear favorite as a proactive, interventive strategy against death. Passive acceptance of the dying process simply does not feel right, whatever the prospects for near-term medical progress, or on the other hand, resurrections in a more distant, technologically superior future. It goes without saying that I also think future life will be worth living—it should be possible to make it so, if future developments can provide the opportunity.