Ranting radical that she might have been, Ayn Rand nevertheless understood the spell of plein air painting. Recently, we came across this spot-on description in The Fountainhead of what it's like to be captivated by the process of painting, to be engrossed, to be in the zone.
"He could not say that he liked to paint. It was neither pleasure nor relief, it was self-torture, but, somehow, that didn't matter. He sat on a canvas stool before a small easel and he looked at an empty sweep of hills, at the woods and the sky. He had a quiet pain as sole conception of what he wanted to express, a humble, unbearable tenderness for the sight of the earth around him - and something tight, paralyzed, as sole means to express it. He went on. He tried. He looked at his canvases and knew that nothing was captured in their childish crudeness. It did not matter. No one was to see them. He stacked them carefully in a corner of the shack, and he locked the door before he returned to town. There was no pleasure in it, no pride, no solution; only - while he sat alone before the easel - a sense of peace."
A little later in the book, the architecht Howard Roark explains that his motive and reward is not the money, not the product, not the fame or the effect on people's lives. What means the most to him is the work itself, the doing. To come up with an idea and then take action to make it real.