Both lines and shapes need to be interesting. They're either thick or thin, dark or light, and so you know that the variance you have causes more interest. Lines cause different moods, and they dominate space differently. They cause direction. Lines are referred to as 'contours', and the definition of a contour is like the outline or the boundary that separates.
Lines divide space into shape.
There's the principle called The Golden Rule. It deals with proportions and has been around since antiquity. It's what the Greeks used when they created The Parthenon. One shape relates to another shape and these shapes relate to the whole. Again, lines and shapes divide spaces.