Leaves have their time to fall,
And flowers to wither at the north-wind’s breath,
And stars to set; but all,
Thou hast all seasons for thine own, O Death!
John Milton

Belief in our mortality, the sense that we are eventually going to crack up and be extinguished like the flame of a candle, I say, is a gloriously fine thing. It makes us sober; it makes us a little sad; and many of us it makes poetic. But above all, it makes it possible for us to make up our mind and arrange to live sensibly, truthfully and always with a sense of our own limitations. It gives us peace also, because true peace of mind comes from accepting the worst.
Deprived of immortality, the proposition of living becomes a simple proposition. It is this: that we human beings have a limited span of life to live on this earth, rarely more than seventy years, and that therefore we have to arrange our lives so that we may live as happily as we can under a given set of circumstances. … It made us therefore, cling to life─the life of the instinct and the life of senses─on the belief that, as we are all animals, we can be truly happy only when all our normal instincts are satisfied normally. This applies to the enjoyment of life in all its aspects.
A sad poetic touch is added to this intense love of life by the realization that this life we have is essentially mortal. For if this earthly existence is all we have, we must try the harder to enjoy it while it lasts. A vague hope of immortality detracts from our wholehearted enjoyment of this earthly existence.
Lin Yutang, The Importance of Living.