WHY WE LIKE SYMMETRY

Many, if not most, humans find symmetry to be aesthetically pleasing. In fact, during the 19th century, it was fashionable to have the left side of a room appear to be a mirror image the right side, even to the point of having the same pictures hung on each side. The dining room in George Washington's home in Mt. Vernon is a classic example of this practice. In a similar vein, cars, planes, boats, and trains are usually designed to be bilaterally symmetrical, at least on the outside, with the left side a reverse image of the right. Those few examples which are not, such as the German Blohm & Voss BV 141 aircraft (pictured below), simply look bizarre to most people (you were warned that the Third Reich produced some weird airplanes).

Bundesarchiv, Bild 146-1980-117-01 / Stöcker / CC-BY-SA 3.0 
[CC BY-SA 3.0 de (http://creativecommons.org/licenses/by-sa/3.0/de/deed.en)],
via Wikimedia Commons 

There is an evolutionary reason why the brain seeks symmetry. The bodies of most animals (over 99% of them, in fact) are bilaterally symmetrical. If you are a prey animal, you want your brain wired to alert to bilateral symmetry in order to spot predators as soon as possible. Conversely, if you are a predator, you are similarly motivated to zoom in on a juicy bilaterally symmetrical meal. 

In short, beauty is not in the eye of the beholder--it is instead in his or her DNA.