>The problem with this is its never ending. A hundred years from now with colonies on Mars someone is going to say the same thing about that little niche over there on Mars that hasn't been explored to their satisfaction.

That's right. However, the more we explore a planet without uncovering any signs of life, the more confidently we can say there probably isn't life there. I'm personally not holding my breath for finding evidence of life on Mars or any other body in our solar system for that matter. There's no evidence yet that I find very convincing.

But it's a large leap from that to claiming with any certainty that there IS no life on any body in our solar system other than Earth. It may seem like we've explored our solar system quite a bit, but we really haven't - not in the detail that would be required to rule out the existence of microbes. To use a common analogy, that's like taking a bucket of water out of a large lake, looking at it with a magnifying glass, and claiming there's no fish living there.

>As much as we want it to be life isn't common.

I'd agree that life as we know it is definitely not common, in the sense that it probably occurs on a small minority of all rocky bodies (most of which don't have any water or atmosphere, so it's hard to imagine any life arising or existing for any geological length of time there). We don't yet know exactly how uncommon it is. Does it arise on 25% of terrestrial planets or moons with liquid water in the habitable zone? 1%? 0.00001% Does it arise in 25% of star systems with terrestrial planets in the habitable zone? 1%? 0.00001%? We don't have sufficient information to rule out any of these possibilities, as far as I know. We can't even analyze the atmospheres, much less the surfaces and soil chemistries, of any extrasolar planets in much detail (or at all, in the case of soil chemistries).

I understand the motivation to tentatively conclude in the negative, so as not to be disappointed. I have a Ph.D. in chemistry and it's still difficult for me to imagine a probable series of steps to life on Earth, so it wouldn't surprise me if less than 1 in 1000 star systems had life. But it's ok (and indeed more accurate) to say that we just don't know how common life is yet.