Augustine in The Confessions of St. Augustine (Christian Classics in Modern English), Reflections, November 23, 1992, p. 41
Sin arises when things that are a minor good are pursued as though they were the most important goals in life. If money or affection or power are sought in disproportionate, obsessive ways, then sin occurs. And that sin is magnified when, for these lesser goals, we fail to pursue the highest good and the finest goals. So when we ask ourselves why, in a given situation, we committed a sin, the answer is usually one of two things. Either we wanted to obtain something we didn't have, or we feared losing something we had.