passion

Gerhard Richter said "One has to believe in what one is doing, one has to commit oneself inwardly, in order to do painting. Once obsessed, one ultimately carries it to the point of believing that one might change human beings through painting. But if one lacks passionate commitment, there is nothing left to do. Then it is best to leave it alone. For basically painting is total idiocy". 
I found this quote here. I have also seen others artists use this on their websites so it must be commonly used.