To me, it seem preposterous and rather self obsesssed to imagine that life has appeared so abundantly here on earth, and yet nowhere else. Even if the conditions required for life are vanishingly rare, that still makes for millions of available planets. Plus of course, it might be that all sorts of different forms of life can evolve in entirely different conditions, although perhaps so different we wouldn't recognise it as life. Outside of our visible spectrum, for instance.