Finish the Song

[Memory of Trees]

I think it's

When winter passed she came again,
And her song released the sudden spring,
Like rising lark, and falling rain,
And melting water bubbling.
He saw the elven-flowers spring
About her feet, and healed again
He longed by her to dance and sing
Upon the grass untroubling.

And that was, as arithron87 said, Aragorn about Luthien.