Whether it's absolutely my favourite love-song depends on how I'm feeling, but one song that never fails to move me deeply is by Ivor Gurney, to a poem by Yeats:

One that is ever kind said yesterday:
"Your well beloved's hair has threads of grey,
And little shadows come about her eyes;
Time can but make it easier to be wise,
Though now it's hard, till trouble is at an end;
And so be patient, be wise and patient, friend."
But heart, there is no comfort, not a grain;
Time can but make her beauty over again,
Because of that great nobleness of hers;
The fire that stirs about her, when she stirs
Burns but more clearly. O she had not these ways,
When all the wild Summer was in her gaze.
O heart! O heart! if she'd but turn her head,
You'd know the folly of being comforted.