"Call it aura. Art always has had to do with aura -- spiritual aura in the past, individual aura in modern times -- and this applies not just to visual art. Religious music, for example, was written to express piety and honor the church. Court music was composed to compliment the king. But in the modern era, secular symphonies arrived. What were they supposed to celebrate? The temperament of the composer. As the pianist Charles Rosen has written, pure instrumental music came to have 'its own law, its own reason for existing: it [was] produced by the artist not for a purpose but because he must -- out of an 'inner necessity'"
Source: The Accidental Masterpiece by Michael Kimmelman