“CLEMENTINE: This is it, Joel. It's going to be gone soon.
JOEL: I know.
CLEMENTINE: What do we do?
JOEL: Enjoy it.”

Context: How do I know that enjoying life is not a delusion? How do I know that in hating death we are not like people who got lost in early childhood and do not know the way home? Lady Li was the child of a border guard in Ai. When first captured by the state of Jin, she wept so much her clothes were soaked. But after she entered the palace, shared the king's bed, and dined on the finest meats, she regretted her tears. How do I know that the dead do not regret their previous longing for life? One who dreams of drinking wine may in the morning weep; one who dreams weeping may in the morning go out to hunt. During our dreams we do not know we are dreaming. We may even dream of interpreting a dream. Only on waking do we know it was a dream. Only after the great awakening will we realize that this is the great dream. And yet fools think they are awake, presuming to know that they are rulers or herdsmen. How dense! You and Confucius are both dreaming, and I who say you are a dream am also a dream. Such is my tale. It will probably be called preposterous, but after ten thousand generations there may be a great sage who will be able to explain it, a trivial interval equivalent to the passage from morning to night.

"Doing What I Know" (song)
Song lyrics
Source: Gilbert O'Sullivan, "Doing What I Know" https://www.youtube.com/watch?v=YlkC9AFrSNU (song on YouTube)

In The Art of Man-making: Talks on the Bhagavad Geeta http://books.google.co.in/books?id=ssyfCDBzOrYC&pg=PA187, p. 187

Source: Journals, 1889-1949

Source: The Winter's Tale

As quoted in Walden (1854) by Henry David Thoreau, Ch. 1
Attributed

Confucius, as quoted in Walden (1854) by Henry David Thoreau, Ch. 1
Misattributed