An interesting subject. I've never really looked into it, but here's an article I googled:
http://www.nimr.mrc.ac.uk/MillHillEssays/1999/handedness.htm.
It seems to say that bilateral symmetry started early in the evolutionary tree. Why and how it started is what I'm not clear about. I think you would have to deduce that bilateral symmetry posed some type of advantage in the process of natural selection by enabling animals to work more efficiently. Ex: It could swim faster with fins on two sides, and a symmetrical body shape.
I think there may be a deeper reason at the cellular level. Similar cells may tend to grow as close together as possible for efficiency, but at the same time provide room for the optimal function of other cells. Two ears on either side of our head probably provide the most efficient internal wiring, while making room for other cells.
Another idea, which is probably less true would have occured later in the evolutionary timeline. Greater sex appeal for animals with greater external bilateral symmetry may have helped shape the evolutionary process. It's true for me at least...
I'm no expert, but just wanted to throw some ideas out. I'd be interested in learning more about it.