Because I can't do anything about it. Even if I'm in perfect health and minimize risks, Doom is still waiting eventually. I don't like that.

And "natural" in this context just means "we haven't found a better way yet". Diseases are natural, but we cure those all the time. Hunger is natural, but we eat. Walking is natural, but we have invented transportation. We do unnatural things like eat cooked food, live in houses, wear clothing, and chat on the internet.

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"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
"The Mayan symbol for "book" looks a lot like a triple hamburger, but I've never seen them claiming it as proof the Mayans had Big Macs." - KaiYeves
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort