First of all, less data is required to build the body – just mirror whatever is on the opposite side. Chromosomes can thus start out simpler. Easier to travel straight – both sides do the same thing = straight forward.

So would it be reasonable to expect any alien life we might find to be symmetrical?

Radial symmetry seems to be tailored for sessile organisms, or organisms that needs to trap or capture their sustenance that moves through or falls on them – they’re like nets and spread, so are efficient if the organism can’t move.

Bilateral symmetry seems more tailored for movement.

They’re not, on the inside.

Cells divide symmetrically!

There’s an interesting discussion on this topic here that reiterates what Rarebear said above- the symmetry is primarily visual.

As many pointed out it’s mainly an exterior thing, as for sensory organs and limbs it makes sense: ears on both sides mean a better 360° hearing, symmetric eyes mean stereoscopic vision and depth perception, symmetric limbs means better stability when moving. All favourable traits.

I read somewhere that all symmetrical creatures have a common ancestor, a type of worm with a primitive brain.

There is speculation that the symmetry of organisms may reflect the geometry of naturally occurring protein structures.

Proteins organize themselves into larger and larger agglomerations, folding themselves in complex ways. As these structures get bigger, they tend toward symmetry. At the largest structures of the heirarchy, oligomers, there is a very high incidence of molecular symmetry among naturally occurring oligomers. Here are two examples.

Symmetrical oligomers tend to form naturally because there are savings in energy to be had by this kind of organization. So while there are many advantages to be had from symmetry on the macro scale of the organism, the original spark that started symmetry in the first place may have had more to do with the way the very molecular ingredients of life are shaped.

You took my answer @Rarebear! :)

I wonder if it has to do with what you need to deal with in life. If you’re moving around and interfacing with other things that move around, it makes sense to be symmetrical, at least in part for the reason @Thammuz listed. Of course, I probably have a symmetry-bias.

Because from an engineering point of view, it’s easier to copy one side over (less genes/code required) – it was also tested by the environment and worked.