Insouciance translates to 'carelessness' or 'nonchalance', but it carries a lighter, more carefree connotation than simple 'carelessness'. It evokes a sense of blissful unconcern, often associated with youth.
In Vianney's song, he yearns for the 'roi de l'insouciance' (king of nonchalance) – his younger, carefree self. He contrasts this with the responsibilities of adulthood, making 'insouciance' a poignant and central theme about holding onto one's inner child.
"Ne Me Changez Pas" is Vianney's plea to stay himself as the world pushes him to grow up and settle down.
The song opens with the pressure of becoming "responsible" almost overnight: bills, rent, a job, all the things that are supposed to make someone an adult. He feels it squeezing the life out of him, especially in a city that never slows down. His answer each time is the same refrain: don't change me, let me keep my dreams, let me keep my poems.
In the second half he turns to talk to the kid he used to be, asking him to come back and look at how worn down his innocence has become. He wants to protect that part of himself, the one that still believes in things, rather than let it get erased by the practical demands of getting older.
It's a warm, gentle song about holding on to who you are underneath the responsibilities life keeps adding on top.