Sometimes it is the subject. But, yes, the most obvious is emotional light. The light attracts me to the subject. Every painting is a success in that I am now beyond it. I have learned something, whether it is something to repeat or avoid next time.
Every painting is a journey and an experience.
Not every painting is a masterpiece. I don't even like them all when they are done, but someone will. The finish is definitely the hard part. All of my creative energy is exhausted, my attention span has been breached, my muscles have atrophied from lack of exercise, and my brain is already onto the next challenge. Sometimes the labor of a painting is the last hour.