Lyric discussion by Satsumo82 

Cover art for Es Mus Sein lyrics by Rufus Wainwright

I agree, the song is related to Kundera's "The Unbearable Lightness of Being". Still there are two possible German translations which make sense when you look at the song or the book. The question "Muss es sein?": "Must it be?" or "Is there an evidence for being?" The Answer "Es muss sein.": "It must be." or "There is an evidence for being".