The long answer is more complex and has to do with how evolution works and survival of the species at the expense of the survival of the individual.
First, let me say that while I tend to anthropomorphisize when explaining things, there is no need to posit any organizing force to explain the results of evolution. For those who would say that the odds of two humans encountering each other such as me writing this and you reading this are so huge that a billion times the life of the universe is not enough time for the event to happen are looking at things the wrong way. The precise series of events that lead to me writing this and you reading this, were they to be replicated, would indeed be of such a small magnitude as to be unbelievable. However, to suggest that our coincidental arrival at this juncture in time is effectively impossible without some guiding force is to ignore that any given series of random events will lead to a highly defined outcome no matter what the route taken. That the exact same series of random events could occur again is quite unlikely, but to see a series of random events that lead to a substantially similar outcome might be quite common. Take, for instance, the evolution of life on Earth. It is likely, based on current research, that life in some form might be inevitable when you have liquid water, minerals including carbon and hydrogen and heat gradients. That our exact form of mammalian existence formed was the result of a nearly endless sequence of random events that never-the-less might, in the fullness of time, result in any number of creatures, a few of which might develop enough intellect to ponder such things. For example, the set of routes from the Washington Monument to the Golden Gate Bridge are nearly infinite, yet no one would be confused if people made the trip. On the opposite side, if you randomly move and turn and eventually wind up making the same trip people would talk about the vanishingly small odds of that happening, but consider: you had to wind up someplace! If you take any series of random events and look backwards in time, it is easy to get the feeling that there had to be a guiding force else how could this or that pitfall be avoided. The reality, though it is often hard to digest, is that ANY such random path will lead from some place to another place and might have narrowly avoided many pitfalls, yet it is still a random path with no guiding force. So, to conclude, though I may anthropomorphisize I am not suggesting any guiding force, I am merely attempting to make a complex subject easier to comprehend.
As I said, evolution is a series of random events. Some events appear to have a disproportionate effect on the long term existence of a species while other events appear to have no impact whatsoever. To say that any species is 'trying to survive' misses the point I tried to make above. Really, there is no ‘species’, there is only a series of individuals making short-term decisions to survive (by eating and not being eaten). The term ‘species’ is a term scientists made up to describe populations of individuals that share substantial traits in common and can interbreed. While I will tell you that there is no hard and fast definition of what a species is and scientists argue about it all the time (note to you creationists), they never dispute that evolution is real and ongoing. So, this collection of individuals all trying to eat and avoid being eaten has arbitrarily been designated as a species by a bunch of humans intent on classifying things. Nevertheless, it is a handy way to describe certain things and as long as you understand the basis for the term, can be a handy way to model real systems. OK, so a random series of events happens to a pool of individuals we have declared a species. What the hell does this mean and why am I spending my time blathering about it? Well, if you look at the interactions of species with other species and the environment they live in it actually becomes quite amenable to mathematical descriptions, hence computer modeling. While the individuals all do things for seemingly random reasons (beyond the eat or be eaten criteria), the species, or the group of individuals, tend to behave in quite predictable manners. For instance breeding. Those who study evolution take as a given that any species with sexes will have a very strong interest and desire for breeding. Why? If the individuals lacked a strong desire in breeding, in a blink of evolutionary time they would have vanished to be replaced by the species whose individuals took greater interest in the act of reproduction. Hence, second only to the desire to eat and avoid being eaten is the desire to procreate. If that were not the case we would not have that species to discuss, unless they are in the tail end of their evolutionary existence as a species.
So a species as a group has a drive to reproduce and make more individuals. The individual’s first priority is to eat and avoid being eaten; the second priority is to breed. At that point the direct pressures on a species may become diffused as there are plenty of ways for the species to thrive. It can lay eggs that, when hatched, contain a new individual capable of surviving in the environment on its own (for instance caterpillars). It might lay eggs that require constant attention prior to hatching, then no attention afterward (alligator). It may have young that require attention for a short time (most birds), moderate time (chimps), long time (humans), intensive (humans again) or just assistance (ducks, quail). It may actually put its offspring in the care of another (cuckoo birds do that). There are all sorts of weird, wonderful and sometimes horrifying means of caring for young (for instance, some wasps inject their eggs into another organism where it grows inside the living being until it eats its way out), but all that is necessary for the species is that the young grow to the age where they can themselves reproduce.
The survival of the fittest is also something that tends to be anthropomorphisized a lot, thus leading to discussions of ‘the guiding hand’. In any population with diversity (in food quality or availability, age, genetics, environment, etc.) there will be certain individuals who are more effective at breeding (or possibly eating and avoiding being eaten; you can’t breed if you don’t manage to survive) than others. Sometimes it is just bad luck as an otherwise optimal specimen might have the misfortune to become lunch for some other individual of some other species. However, for whatever random reasons, certain individuals will breed more than others and over a long period of time (or not; in small populations undergoing a lot of stress, this might happen in just a handful of generations (where generations might be measured in months)) those better breeders will put their stamp on the species by becoming over represented in comparison to the not-so-great breeders. This is now new species evolve, by the way. When two populations within a species undergo enough of these random divergent events that they no longer interbreed, by many (but not all) definitions these individuals are now two separate species. Thus, survival of the ‘fittest’ is really an incorrect term (though very handy if you understand the underlying assumptions, just like species), best breeders due to random events is the better description (though not as catchy, I think).
OK, you are saying (if you have stuck with me this long), what does all this have to do with dying? Well, lets say we have a species that, for some reason, has evolved immortality (not that they can’t die, but they don’t grow old, thus barring accident can theoretically live forever). If the species continues to breed, at some point the environment can no longer support the population and they start to die of starvation and their ‘immortality’ is irrelevant. If they stop breeding (or are breeding at just the rate to maintain the population level), they also stop evolving (or slow it to the point where it effectively stops). At some point there will be some confluence of random events that will lead to the immortal group of individuals (can’t really call it a species if it doesn’t breed) to individually die from accident (something learns to eat them, the environment changes, an asteroid smacks them, etc.) and bingo, you got no more individuals left, hence no chance for a species to evolve. Thus, I am convinced, that had any immortal group evolved, in a blink of geological time they would have been snuffed out by their breeding companions. Thus, a long winded way of saying that dying is just Nature’s way of making room for the next generation.
Let us consider, now, the thought that humans get smart enough to make themselves immortal. The most immediate initial consequence is that our population growth rate would essentially become infinite immediately. Say that the average lifespan now becomes 10,000 years (meaning that of any population, half of them have died of accident (run over by a truck, choked on a bone, slipped on a bar of soap, etc.) within that period of time). If the breeding rate is such that more than one person is added to the population in that 10K year interval the population will continue to grow until the environment can no longer support the number and people start to starve to death (thus making a mockery out of the 10K year average life expectancy). If, on the other hand, we somehow restrict our population such that no more individuals are introduced than die by accident, we can live within our environment’s ability to support us, that is until the environment changes, we get smacked by an asteroid, invaded by body snatchers, etc.
Of course, if we get real good at geo-engineering, keep a weather eye out for the body snatchers and steer all asteroids into safe orbits, we might be able to stave off the inevitable extinction of our species for a very very long time (millions of years, perhaps more). I do think that, barring the killer asteroid or some such, that in the next 50 years or so we will achieve practical immortality, at least insofar that we can live long enough to take advantage of the next breakthroughs in longevity. Still, since we will effectively stop evolving, that means we are destined, in the long run (which, as I said, can be very long indeed) to be overtaken by some species that lacks immortality.
So, what will you choose when you have the chance at immortality? Your own individual future or the future of the species? My choice is simple, I want to live forever.