For the record, I don't repaint things very often. Sometimes I just feel like the emotions I had while painting it the first time have changed so drastically as to make the art seem "wrong" to me.
This is especially interesting given that it's important to me that each painting be reactive to all types of light, and change throughout the seasons. I guess it goes deeper than that too, as I occasionally want (need) to alter something to reflect who I am at a later time, and how I've changed.
When a painting is purchased by a collector, the piece feels done to me. I have no desire to change it at that point. Once someone else has found something in it so profound that they want to own it, to have it in their own space, it seems to me that the process by which the art came about is complete. I make my art to put out in the universe. When it has a purpose higher than myself, an importance in someone else's life, it no longer belongs to me.
This might be an example of my crazy artist brain. I'm not sure. Some artists outright destroy their own work. I do know that I value my right as an artist to do what I want with my own art. There is the possibility I will repaint something on a whim if I suddenly feel the urge to do so. In this sense, it's good to get the painting away from me, out into the world so that I no longer feel any power over it, or powerlessness to the emotional struggle it causes me.
Either way, it's all part of my internal process in life. I view it as a good thing.