[cmsimike]

I think this goes back to searching for other life on earth-like planets. We know what life looks like on our planet, so that's really the only thing we have to go off of, otherwise we are stabbing in the dark.

Likely we are searching for radio waves and light since that's the only thing we know how to communicate with.

[bluGill]

It is more than that. We know what life looks like: composed of carbon because that is the only atom than easily forms long chains of double bonds. It is also very likely to be composed of the atoms that are common in the universe: the low Atomic number ones.

[gbrown]

Also, liquid water is very convenient for complex chemistry.