Ichi-go Ichi-e

Ichi-go ichi-e (一期一会) is a Japanese idiom which literally means "one time, one meeting." It's the idea that each encounter is unique and fleeting, "of treasuring the unrepeatable nature of a moment" (Wikipedia).

It reminds me of a late-19th-century poem by AE Housman, From Far, from Eve. I especially love the second stanza:

Now—for a breath I tarry
     Nor yet disperse apart—
Take my hand quick and tell me,
     What have you in your heart.