And lost he wandered under leaves,
And where the Elven-river rolled
He walked alone and sorrowing.
He peered between the hemlock-leaves
And saw in wonder flowers of gold
Upon her mantle and her sleeves,
And her hair like shadow following.
Enchantment healed his weary feet
That over hills were doomed to roam;
And forth he hastened, strong and fleet,
And grasped at moonbeams glistening.
Through woven woods in Elvenhome
She lightly fled on dancing feet,
And left him lonely still to roam
In the silent forest listening.

He heard there oft the flying sound
Of feet as light as linden-leaves,
Or music welling underground,
In hidden hollows quavering.
Now withered lay the hemlock-sheaves,
And one by one with sighing sound
Whispering fell the beachen leaves
In the wintry woodland wavering.

He sought her ever, wandering far
Where leaves of years were thickly strewn,
By light of moon and ray of star
In frosty heavens shivering.
Her mantle glinted in the moon,
As on a hill-top high and far
She danced, and at her feet was strewn
A mist of silver quivering.

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.
Again she fled, but swift he came.
Tinúviel! Tinúviel!
He called her by her elvish name;
Lúthien Tinúviel was exceptionally beautiful and graceful because she was the only child of Elu Thingol, one of the greatest of the elves, and the Maia (minor god) Melian. 
 Lúthien, as half-elven, half-maia, was said to be the fairest and most beautiful of all beings in Middle Earth. 
 Given that Tolkien based her off his wife, it makes sense he would heap such high praise on her.