Tears.

She danced in the night as the rain fell down
How could you dare forget me?
They smiled, hushed, as the rain fell down, and then she started singing.
She sang of love, of joy, of death, and how those three connected, and hard they tried, although they knew they'd never understand it.
How could they know of what she spoke?
Never had they seen it, and while she sang, those smiling eyes turned dark with tears of anguish.
"Live on," she hummed, "live on, my love," though well she'd say she knew it, her love had died a few years back; forever they'd be parted.
And as she cried, the group looked on, their eyes a-filled with wonder, for hard they tried, but knew quite well, they'll never know what plagues her.