Why humans prefer symmetry

A highly symmetrical face leaves little work needed to be produced by the perceiver. The perceiver essentially does not find "obstacles" in what hie perceives, which has the inherent property of requiring less work. Not inciting work keeps the perceiver's synchronisation with hier's environment stable -there is no perturbation and consequently no change. This non-change then is experienced as trygghet (safety, security, trust). Trygghet being a basic human need means it will be preferred.