Full Idea
Wouldn't you think a man a prize fool if he burst into tears because he didn't live a thousand years ago? A man is such a fool for shedding tears because he isn't going to be alive a thousand years from now.
Gist of Idea
It is as silly to lament ceasing to be as to lament not having lived in the remote past
Source
Seneca the Younger (Letters from a Stoic [c.60], 077)
Book Reference
Seneca: 'Letters from a Stoic (Selections)', ed/tr. Campbell,Robin [Penguin 1969], p.127
A Reaction
These thoughts are traditional, dating back to Epicurus, but Seneca is exceptionally going at finding new variations and examples to reinforce the basic thought.