Most of us are visual creatures, even if we don't realize it, and we have an innate tendency to seek out patterns in our surroundings. After all, if you were living in the savannah of Africa as our ancestors were, your survival could depend upon whether or not you recognized that all the prey animals were avoiding a particular stand of trees because a leopard had just moved in there.
We also have a tendency to be attracted to visual harmony and it can make a painting attractive to a viewer, even when there are faults in the execution. There are many factors to be considered when setting up the composition of a picture. One way to create harmony in a painting is to repeat the same lines of direction throughout a composition. If directional lines, such as the edges of objects and areas of contrast between light and dark, run parallel to one another, then it will give a far more harmonious feel than if those same lines point in many different and random directions.
In the painting shown here, most of the parallel lines of direction point from the top right to middle bottom left of the image. The line is repeated many times throughout the composition, creating a repeating pattern that gives a feeling of harminy. I have found that the slope of a line relative to the horizontal or perpendicular affects the message being sent by the composition. This is a portrait of love and contentment, so the lines are nearer to straight up and down, where a diagonal drawn from corner to corner would imply more energy.
You're not usually going to have all lines pointing in the same direction, because then there is nothing to hold the viewer's eye within the composition. To pull the viewer back into the picture, I chose lines running from top left to bottom right. They form a gentle angle to the first set of lines and do two things:
1. the angle of intersecting lines creates tension. The greatest tension is when lines are at right angles to one another. In this painting the lines intersect at a wide angle, giving the feeling of harmony.
2. the intersection of some of the lines create cups in which important features of the painting can rest. In this case it is the chin of the woman and then her elbow, which is not present in the picture but would be a natural projection of the two lines following the outside of her arm.