Beauty, whether objective, subjective or relative, is all about Selection! What we like, whom we wanna mate with, what we wanna eat, where we wanna live, how we wanna project ourselves, etc. are all abstractly defined by this perception.

In a way beauty is an interesting amalgam of symmetry, consistency, variety and a slew of other contextual features that help us choose the best almost instinctively.

Symmetry symbolizes genetic health / structural strength. We hardly cherish asymmetric faces and objects. Consistency can also be seen to work in the same context. Inconsistent color for objects, skin tone, etc. infer a poor genetic / systemic health and viability.

On the other hand variety is an interesting facet of beauty and encourages diversity! We are drawn to faces that are slightly different from those of our herd/group and this behavior can be seen as an impetus to prevent inbreeding and encourage genetic diversity.

Finally, there are special features we associate with beauty in specific contexts. Women with large breasts (nursing capability), slender waist (unlikely to be pregnant), etc. and men with a muscular body (ability to defend and provide), sharp chin (testosterone levels and fertility), etc. are desirable for the above mentioned reasons and these affinities could have been acquired through natural selection.