That is one of the hardest questions an artist often has to answer, not only to a random bystander, by to himself as well. This happen can happen to great masters, as shown in the play "Red", when Mark Rothko struggled to understand where his paintings stand and if they were not completed, what went missing. Admittedly, it was a fictional account of Rothko's life, but it rang absolutely true.
As for me, it had happened many times. Sometimes, the dissatisfaction could take place several years after the "completion", or just few hours after the apparent completion.
Recently, I completed an oil painting - Orestes, which was another abstract rendering on the theme of a well-known Greek tragedy.
I saw Orestes as a bunch of white lights, emerging from dark elements, the purple and black paints, which signify the unrelenting furies. On the top right of the canvas, I used some lightly applied yellow stokes to indicate the purity and light Orestes tried to achieve, or the wisdom of the god and goddess. Happily, I signed my name on the canvas. Fin!
When I photograph this piece, I became less happy with those yellow stokes and decided to balance them with some light blue paints, and incidentally, I could use them to differentiate Apollo and Athena. Once more, I believed that the painting was finished.
A few hours later, I felt dissatisfied again. This time, it was those white paints annoyed me. The trajectory was wrong. I bent them over a little bit, towards the right, meeting the yellow and blue streaks head on. I also added scattered white drips, to indicate more sense of motion and struggle. Finally, it was really finished.
Oil on Canvas
14" x 11"