Wow! You sure know how to ask the tough questions! In my opinion, I think the main reason why life must end for any organism - plant, animal or bacteria - can be found in Isaac Newton's Second Law of Thermodynamics, which says that all organized systems tend to fall into disrepair with time. So, no matter how much effort you put into maintaining your health, the organized systems in your body that you depend upon (nervous system, circulatory system, respiratory system, etc) will start to fail you eventually. No process which requires energy to maintain itself can last forever. Machines break down, life ends, even the sun in our solar system is slowly dying out. Some reasons why our bodies cannot exist forever are very obvious. Cells in our body are "programmed" to die after a certain time. Sometimes they are replaced, but sometimes they are not. For example, skin cells are constantly dying and being shed, but they are continuously being replaced by new skin cells just beneath the old ones. Our brain cells are also dying, very slowly, but unlike skin cells, brain cells cannot be replaced.
How cells are triggered to die, what controls how long different cells live, why certain cells can be replaced and others cannot, where in our DNA all this information is stored...all these questions are being studied by scientists but are still unanswered. The fact remains that as we age our body slowly goes into disrepair: our bones and muscles weakens, our immune system is less able to protect us from disease, we tire easier, our senses become less acute (hearing fades, eyesight becomes poor, etc.), we heal
slower, etc. Can you think of any advantages to having cells that are programmed to die, if they are replaced with new cells? (How do you think a caterpillar turns into a butterfly?)

In a general sense, a organism's life span is related to its size. This holds pretty well for mammals, at least (a group that includes humans). For example, a mouse or a hamster lives less than a year while a dog can live up to 15 years and a human over 100 years. This is because the larger you are, the slower your metabolism runs. Your metabolism is the general rate at which your body's systems work. A mouse's heart beats 6 times per second and a mouse can digest a meal in a few minutes while a human's heart beat
about once per second and it takes us several hours to digest a meal, so we say a human has a slower metabolism. The slower your metabolism runs, the less energy you spend maintaining it and the longer it will last. As humans, we have managed to increase our average lifespan a great deal (from less than 40 years in the 1600's to 70 years or more today). By protecting ourselves from disease, improving our food supply, and using medical technology we have removed some of the major threats to our health. We have also added a few new ones!

Someday scientists might be able to remove the information in our genes that tells cells when to die, or somehow add information that will enable cells in our body to repair themselves or prevent damage from occuring. One recent discovery suggests that oxidative damage (damage caused by oxygen) to specific cells (motor neurons) may play a large roll in the aging process in humans. We have a special enzyme in our cells that helps prevent oxidative damage, and perhaps we can use this enzyme to slow down the aging process. So we may be able to maintain our eyesight and muscle strength a little longer. But until we figure out how to get around Newton's Second
Law of Thermodynamics, I believe our bodies will always "have to die" at some point. I am sure some people would disagree with me.

A scientist answering this from a scientific perspective might say that cells can only divide a certain number of times before they don't work quite right any more.In other words, it is as if they have a built-in clock that eventually runs out. Even if we are not growing, cells in our body die and need to be replaced by new cells, so the process of cell division is still very important. This is controversial, but I think there is evidence to support this idea of limited cell division.

On the other hand, even as a scientist, I am not sure that science has the best answer to this question!

I'll acknowledge that death of humans is an emotional topic--just last month I literally watched my grandfather die and a few years earlier, my father. Although it would be nice if nobody would die, death is an obvious reality.
Some reasons for dying have no rational explanation (i.e. Colorado). So I'll stick to the ecological reasons for the concept of death and not the process or the philosophical meaning. I'm sure your class has heard the Disney explanation of the "circle of life" and there is an element of truth behind the marketing. Imagine that you have a virus or a bacteria that causes no disease but where the individuals cannot be killed. Although the size of any individual seems to be harmless, any individual requires resources such as space or energy. As the population grows and grows, they consume these resources to the detriment of all other life (remembering that energy cannot be created or destroyed). Resources are not available for other life forms. Some time in the future, the virus or bacteria will cover the planet and will have crowded out all other life. Death is necessary to allow resources to cycle between organisms so that different SPECIES can continue to survive rather than just INDIVIDUALS.

Tough question... why does any living thing die? Scientists and philosophers alike have pondered this one for ages. Consider a finely built clock. Its builder tends it daily with loving care, rewinding, oiling and polishing. He passes on his love of the clock to his son, who tends it daily and eventually even has to begin to replace parts. It's not because they were damaged, but just that friction and gravity eventually wear down the metal. The son passes on the tending to his daughter, who passes it on and so on and so forth. Finally, 8 generations later, there are no more repairs possible, the mainspring has just finally given out. Not through neglect, not through damage, but just use. It's the same for any machine. It's the same for a piece of granite-- eventually it turns to sand.
So what about living things? Well, I can't answer "why humans HAVE to die," but we can think about why human (and other ) living bodies "wear out." In many ways, an animal's body is a machine -- the heart muscle pumps, the blood veseels and capillaries are the hoses, the muscles and bone are the pistons, the liver and kidneys are the filters, etc, etc... All of these physically wear down or clog up. That is one reason why bypass and other heart surgeries are so common. One really interesting aspect of "aging" that was recently discovered has to do with the DNA in each cell of our bodies. Without getting too detailed, each chromosome has lots of special sequences called TELOMERES at each of its ends. These sequences ensure that the genes on the ends of the chromosomes are faithfully replicated during each cell cycle. But the consequence is that some telomeric sequence is lost in each round. Eventually, with time, replication is no longer complete and important genes may not be replicated or expressed properly. So, even the DNA can "wear out."
This was one of the main questions scientists wanted to know about Dolly, the sheep that was cloned. Do you remember what they did? First, the scientists took a sheep oocyte (egg) and they removed the nucleus. Then, they were able to put the nucleus (DNA) of another sheep into that egg, and then stimulate development. The result was Dolly. So even though Dolly is herself only a few years old based on her birthdate, her DNA is actually much older (it is Dolly's age plus the age of the donor). So how old is
Dolly really? And will she "age" much faster? Scientists are looking at this question pretty closely and the answer is still unknown.

Biologists look at two types of answers to a question like this. One is how exactly does it happen. The other is why a particular thing (mechanism) might have evolved. Let's tackle the first question first.

We replace old, damaged or dead cells through cell division (mitosis). Some cells, like nerve and muscle cells, never divide. That's why when we have an injury to our brain, spinal cord, or heart (what is the heart made of?), these parts do not recover well. Other cells die or are damaged through normal use. As we get older, all of our cells divide less. This means that our parts start to "wear out" because the cells that make them up aren't replaced as fast. Each cell is programmed to only divide a certain number of times. Cells that don't have this built-in message to stop dividing cause tumors or cancer because they become unspecialized (no longer do their proper jobs) and at the same time they divide out of control, taking over the space of normal cells. Things with fast metabolic rates (which you can tell by their heart rate) seem to "wear out" sooner, so shrews are dead of old age in 2 years while tortoises may be over 100.

So why do we have a "timer" on our cell division? Why doesn't each person live forever? When we want to answer a question like that, we look around at what lives for a long time and what doesn't. Among animals, things that are big and/or smart tend to live longer. (why?)Let's do a thought experiment. What if every person who had ever been born were alive today? Things would be a bit crowded, right? In fact there wouldn't be food, water, or space for all of us. Now let's say we take anyone who died from a cause not related to aging out of the picture. That would still leave a huge number of people, more than the earth could support. There certainly wouldn't be room to add new people.

Let's scale this way back to the individual level, where selection takes place. Things that survive and reproduce leave more offspring that have some of the same traits that made them successful. The recipe for the traits are in the genes they pass on. Why don't we see animal species where an individual keeps on having offspring and neither parent or offspring die of old age?

In a species that reproduces sexually (whether plant or animal), a parent gives its offspring half its genes. (Where does the other half come from?) So from an evolutionary standpoint, having two children alive is as good as being alive yourself if it comes to passing genes. The math shows you will leave more genes by having 3 offspring and dying than by living. The offspring will have to compete with the parents for resources unless there's some other place to go where there are more. Plus, a random danger like a falling tree could wipe out the whole gene collection if only one individual had them. If the genes are in two individuals, only half would be lost. So maybe having offspring, then dying means you leave more genes than you would by hanging around and competing for resources. And if other problems, like injuries and disease also make you less and less able, your two offspring might do better than you.

For the really long-lived things, we have to look at plants, which might live thousands of years. Even the longest lived animals such as people, tortoises, parrots, and elephants don't live anywhere near that long. How are plants an animals different? Well a plant might send thousands of seeds long distances away, presto, no competition with its "parent" plants. Plus, if a part of a plant is detroyed, a new piece can grow in its place, so there's less need to "start over" with a whole new plant. Last, few plants actually survive the sprouting and early growth part of their lives, so it makes sense for the successful gene carrier (parent) to hang around rather than take the long shot that two of its seeds can successfully replace it.

Many cultures have struggled with the question "why do we die?" and come up with other answers, which may involve one or more gods or spirits. Many people find comfort in the non-science explanations. In science we can only look at processes that obey natural laws. This doesn't mean other explanations are wrong. They're just not science.

I don't think anyone really understands (natural) death in a scientific way. Why cells die and why living things age and die is still a mystery as far as I know.