When Kurt was excited, his voice was tutti, all bright and happy and sweet.
When Kurt was angry, his voice was staccato, short, harsh, abrupt.
When Kurt was tired, his voice was legato, smooth and flowing.
Puck knew all Kurt's voices, from the dead tone he used with Rachel, to the accidental-filled vibrato he used in the bedroom.
He loved them all, but most of all he loved knowing he could make Kurt sing the sweetest song of all, where the tempo changed from allegro to maestoso and the crescendo was the whispered promise, those three words. I love you.