[mikeash]

Could be, but historically things have leaned towards "we are not special" and it seems like a good working assumption to continue with that. Despite previous thought, we're not really any different from animals, animals aren't really different from other forms of life, living matter isn't really different from non-living matter, our planet isn't really different from other planets, our star isn't really different from other stars, etc. That obviously doesn't prove that life is a common thing, but in the absence of better info, it gives a decent starting place.

There's also the fact that life arose on Earth almost immediately after conditions allowed for it. If it were an extremely rare event then there likely would have been some lag. Again, not conclusive, but suggestive.