It depends what you belive in but basically I have read once scientific paper that explained that we are dying because our cells start losing water and eventually lose ability to absorb water and as AN effect of such process they lose they ability to multiply. However I have read few other studies that explain the effects of the death and few seemed possible to me.

First of them was explaining that each cell in our body and not only our body but in fact every other leaving being body. Is able to eat certain sort of energy and when this cell becomes distorted then it starts eating other cells energy and eventually other cell energy losing it main food and it becomes distorted making the thing that first cell is cnacer then eventually all of these cells are deforming and eventually person is dying however this explanation is only connected to the cell canceer and not to the dying of all age. They had also provided solution check which cell is eating which energy and then feed it with that energy this would effectively cure cnacer. Russian scientist are still doing experiments on this field hopefully they will find something soon.

If u succes to answer of "Why we die from old age" talk to me, if any1 knew why, there probably was a cure for that.

Long story short:

We die because of numerous processes that take place in cells. Replication itself is a way of "dying a little more":
If you read the huge quote back there, you may have noticed that when mitosis is performed, DNA is doubled. For that DNA doubling, some little enzymes called polymerases are needed; they stick themselves onto the original DNA, and start copying from that position forward... However, they don't copy ALL the DNA... The polymerases are not able to copy the last few nucleotides from the strand and as such, the DNA strand's replications are smaller and smaller. Lucky for us, the information present in those tips is just junk, mainly repetitions of the same nucleotides... However, when we reach an age where those fake bits of information (telomeres) disappear, real information starts to be left out of replication, rendering the cells useless. In many cases, they may begin to behave strangely, due to protein malfunction, etc.. This is one reason for aging and dying.

Another reason is the production of reactive oxygen species... As some of you may know, cells need energy, and that energy is obtained through ATP. Well, the process through which this ATP is produced is highly dangerous to the cell, because it oxidizes proteins, lipids, etc. This oxidation leads to protein malfunction, etc etc.

The very glucose circulating in our blood (which is the essential substrate for energy production) may ruin proteins and other components of cells. In fact, it does that (there are some papers regarding the matter... protein glycation :))

Well, that's about it. Living makes you die. The oxygen you breathe makes you die. The food you eat makes you die. However, you get to live, first. Also, you can only die if you live! And a life without death isn't really a life ;)

I saw this one video where it kind of explains why this is. Take a photocopy of a picture for example. The original picture of course has the best quality, no artifacts, etc. Now when you make a photocopy of that, the picture isn't going to have the same 100% quality as the first. Of course the quality is going to degrade a bit. Now make a photocopy of that first photocopy - the picture gets worse. I think cells work in the same way. When we're young - are cells are at their prime. As we get older, they don't work the same as they used to.

Your chromosomes have things on them called telomeres. Each time they replicate during cell division, the telomeres on the new chromosomes shorten just a little bit. Eventually, after many replications, they are gone. This is when the cell "knows" not to reproduce anymore, and is part of how we age.

One cause of cancer is telomeres that don't shorten as they should. As a result, the cells keep replicating and replicating.

I think we die, cause I heard this professor talking about how the DNA, over time, doesnt replicate as efficiently as before, so Over time, the DNA gets contaminated and less potent, until we become old and the cells aren't as efficient as they were.

Anyone want to go with me to the fountain of youth? That's one surefire way to stay young forever!

As I said before, DNA is not the only factor that accounts for aging. We have to think about A LOT of variables, such as endocrine axis, protein malfunction, etc etc... Summing it all up as "DNA replication defects" is not a very accurate definition for the process
I've got some cool papers on the subject... If anyone wants to know a little more about it, I'd be happy to send you the links to them (however, I do not guarantee that they're free )