Who has not wanted, does not guess
What plenty is.--Who has not groped
In depths of doubt and hopelessness,
Has never truly hoped.--
Unless, sometimes, a shaow falls
Upon his mirth, and veils his sight,
And from the darkness drifts the light
Of love at intervals.

And that most dear of everything,
I hold, is love; and who can sit
With lightest heart and laugh and sing,
Knows not the worth of it.--
Unless, in some strange throng, perchance,
He feels how thrilling sweet it is,
One yearning look that answers his --
The troth of glance and glance.

Who knows not pain, knows not, alas!
What pleasure is.--Who knows not of
The bitter cup that will not pass,
Knows not the taste of love.
O souls that thirst, and hearts that fast,
And natures faint with famishing,
God lift and lead and safely bring
You to your own at last!

James Whitcomb Riley

http://www.poemhunter.com/poem/unless/