Here's my answer to the very real existential crisis that grips me midway through everything I've ever tried to do: I think stories help us fight the nihilistic urges that constantly threaten to consume us.
Top 10 Fighting Quotes by John Green
The prospect of a world that contains neither humans nor Z's is not so terrifying. Nature will take its world back. Animals will frolic and fight. There will be no lord of the manor, which is not such a bad thing, because it seems to me that people have done a pretty poor job of guiding the biosphere for the last few thousand years.