“The fear of death follows from the fear of life. A man who lives fully is prepared to die at any time.”
Not by Twain, but from Edward Abbey's A Voice Crying In The Wilderness (1989).
Misattributed
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Mark Twain 637
American author and humorist 1835–1910Related quotes

From 1999 interview.
Noted in the October 2003 BBC News profile of Ebadi. http://news.bbc.co.uk/2/hi/middle_east/3181992.stm (retrieved Oct. 15, 2008)
“Don't fear death, fear the un-lived life”
Variant: dont be afraid of death, be afraid of the unlived life.
Source: Tuck Everlasting
1 December 1982
The Teachings of Babaji. (1983, 1984, 1988). Haidakhan, U.P.: Haidakhandi Samaj.
Source: The Teachings of Babaji, 1 December 1982.
Fire from Heaven (1969)
“It is not death that a man should fear, but he should fear never beginning to live.”
Marcus Aurelius, p. 225
They Both Die at the End (2017)