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 ;)
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