Never give all the heart, for love will hardly seem worth thinking of...

For everything that's lovely is
But a brief, dreamy, kind of delight.
O never give the heart outright,
For they, for all the smooth lips can say,
Have given their hearts up to the play.
And who could play it well enough
If deaf and dumb and blind with love?
He that made this knows all the cost
For he gave all his heart and lost.

-- W. B. Yeats