Circles
Research shows that people prefer circles over angular shapes, as they associate angular lines with anger and agitation. Curves are softer and more gentle. Circles remind people of feelings like tenderness, affection and care.
Soft Curves
Other shapes with curves like ovals, for instance, or soft curves, subconsciously convey rhythm, femininity and pleasure. These are usually more appealing than shapes with straight lines.
Symmetry
People are naturally drawn to symmetrical shapes, and this also applies to physical attractiveness. Therefore, perfectly-symmetrical shapes like squares and rectangles are more appealing than irregular polygons. This preference is based upon the fact that humans are symmetrical -- we have two ears, eyes, arms, etc. Symmetrical shapes suggest balance, stability and reliability.
Horizontal Lines
Horizontal lines also suggest femininity, as well as peacefulness, calm, and silence. Shapes with zigzag lines can be assumed to be aggressive, while horizontal ones are non-threatening and composed.