Robert Fripp on the mystery:

For me art is the capacity to experience one's innocence. Craft is how you get to that point. Maturity in a musician would be the point at which one is innocent at will. At that point the relationship between the music and the musician is direct and reliable. 

The relationship with the music is always mysterious. When it works you can never tell, you can never guarantee that it's going to work. You can only put yourself in a place where it's more likely to happen.