Symmetry, especially facial symmetry, is one of a number of aesthetic traits, including averageness and youthfulness, associated with health, physical attractiveness and beauty of a person or non-human animal according to the authors of Facial Attractiveness: Gillian Rhodes, Leslie A. Zebrowitz. It is also hypothesized as a factor in both interpersonal attraction and interpersonal chemistry. [Wikipedia]
Human beings always pursuing beautiful things. What does beauty mean to you? IMO, the beauty is health. I guess, that emotion is for healthy breeding instinct.
A simple example is a facial symmetry.
We all know that facial symmetry is a important factor in human beauty, many researches have shown that lack of facial symmetry on average correlates with lower beauty rankings. Why Human beings feel beauty from symmetry and balance? AFAIK, the asymmetry of face is affected by spinal curvature. It means that they are unhealthy.
I've never seen animals who have asymmetry of face in the wild nature. There is a rule of jungle - "removal of the unfit and unhealthy".
See also: Beautiful people are more intelligent