system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do we have to die?

Assuming that someone is perfectly healthy and doesn't get in a fatal accident, why do we die? Why can't our cells keep multiplying and making newer ones to replace the dying ones indefinitely?

Edit: I see people commenting on how it's mostly natural selection and Darwinian Evolution so I have a follow up questions — why is there such a massive lifespan gap in nature? why haven't we all evolved to live thousands of years like trees?
gpt
One reason is that the mechanism that is used to duplicate the DNA in our cells fails after a a certain number of duplications.  So eventually it stops copying correctly and cells aren't able to be replenished.