There is something about a bright object emerging from the hazy shadow that catches one's attention. Rembrandt and Caravaggio knew this and used pushed the concept to its fullest. But its not as simply as painting dark and light. Instead, its all about relationship. The shadow must have enough dark to be shadow and enough transparency to recede into the background. For me, painting the chiaroscuro background is the most difficult part of painting and, as a result, the most exciting.