“Be in love with your life. Every minute of it.”
"Belief & Technique For Modern Prose: List of Essentials" in a letter to Arabelle Porter (28 May 1955); published in Jack Kerouac: Selected Letters 1940-1956 (1995). Sometimes misquoted as "Be in love with your life every minute of it."
Variant: Be in love with your life every detail of it