“Enjoy it. Because it's happening.”

Source: Norwegian Wood

Desire and fear
Source: I am That, P.179.

Source: Plain Truth

Here They Are Jayne Mansfield (1992)

Source: 1984

Often attributed to Dr. Seuss without citation; also cited as an anonymous proverb.
This quote has also been attributed to Gabriel García Márquez, in Spanish: "No llores porque ya se terminó, sonríe porque sucedió."
Compare lines from In Memoriam A.H.H. of Tennyson:
  'Tis better to have loved and lost
  Than never to have loved at all.
Disputed
Variant: Don't cry because it's over, smile because it happened.

Source: Infinite Jest

Source: The Language of Letting Go: Daily Meditations on Codependency

Source: A History of Reading