most animals are essentially tubes with a mouth on one end, a gut in the center, and an anus to shit all over everything (aka a worm). the asymmetry between front and back i believe stems from waste materials often being toxic to an animal, and how basic locomotion moves you in one direction.
when animals got big enough, gravity introduced a new asymmetry and large animals got a top and a bottom to adapt.
however, nature hasn't presented much of an asymmetry to help us differentiate left and right. most asymmetry is internal to bodies, probably due to limited space and the need for particular organs.
thus, most animals have bilateral symmetry, or that's how i reason it out. I image complex animals on other planets would prefer this symmetry too
