The title of this post is surely one of the biggest questions for which we have no answer. Of course it is one of those questions - like "Does God exist" - for which we shall never know the answer if it is "No" but could only know the answer if it is "Yes". Put another way, the hypothesis that there is no extra-terrestrial life can be falsified but its opposite can't. Or to paraphrase yet again: the null hypothesis is that there is no extra-terrestrial life. I labour this point because often in science one's position is to accept the null hypothesis until we know otherwise (are there fairies at the bottom of your garden) and yet the scientific and popular literature is full of assertions about the overwhelming likelihood that life elsewhere exists.
The basis of these assertions is almost always the famous Drake equation which is an estimate for the total number N of intelligent civilisations in our galaxy based on multiplying together several unknown quantities. In 1961, when Frank Drake invented his formula, we had virtually no idea about planets beyond our solar system. But over 50 years later we now know that they are very numerous and we therefore have a much better idea about some of the factors in the formula. This knowledge has made some people very hopeful that not only is N>1 but that N might be very large. To put it crudely the argument goes: "Since there are literally billions of planets it is exceedingly likely that some of them will contain life".
I don't accept this argument. The Drake equation contains another factor that measures the probability of life beginning (and I mean beginning, not evolving) on a planet with the conditions to sustain life. We know nothing about this probability. It may be so small that, even though the number of planets is vast, our own existence may be just a one-off fluke. I am not saying that no extra-terrestrial life exists; I am saying that we do not know enough to put an estimate on how many instances of it there are, and in particular we cannot say whether this number is likely to be greater than 1.
Why then is there such a lot of firmly-held opinion on the question? I think the belief in alien life is partly the result of wishful thinking. It's a cool idea that there may be beings elsewhere in the galaxy, one that has inspired very many science fiction stories. Wouldn't it be sad to consign all that imaginative writing to the fantasy bucket? At the end of this post I'll advance another possible explanation for this "belief without evidence".
By the way, we have a much better understanding of how life can evolve once cellular life forms exist so the "trick" might be to explain how readily chemistry can give rise to very complex molecules. However, that is not the only hurdle we have to vault: some scientists believe that other rare factors enabled the development of life on Earth (such as the shielding effect of Jupiter's gravitational field as an asteroid deflector).
But, before that, let me just touch on some of the scientific estimates for how much extra-terrestrial life there is out there. First we have Andrew Watson (reported here) who, in 2008, devised a model that suggested that the probability of (intelligent) life evolving anywhere else is less than 0.01%. Watson's model took into account the steps from the formation of single-celled bacteria through to intelligent life with an established language. I haven't managed to access his original paper but I think it must be problematic to estimate the chance of replicating molecules arising.
Next we have Edwin Turner and David Spiegel writing in the Proceedings of the National Academy of Sciences in 2012 (and reported here). They too are pessimistic that life exists elsewhere and they specifically addressed the objection "But it exists on Earth, so why not elsewhere" using a Bayesian analysis.
In the other corner we have an opinion poll: 85% of those who answered a recent Debate.org poll voted for extra-terrestrial intelligent life. Most of the believers do not advance serious arguments. Some scientists, like Stephen Hawkins, merely make the claim that alien life is extremely likely (see here). Or Harvard physicist and SETI leader Paul Horowitz. He stated in a 1996 interview with TIME Magazine, "Intelligent life in the universe? Guaranteed. Intelligent life in our galaxy? So overwhelmingly likely that I'd give you almost any odds you'd like." These types of statement are extremely common. They stem from a knowledge of how many planets there seem to be but ignore the difficulty of life getting started.
I would like to advance another reason why some scientists and many non-scientists are prone to advance the unsubstantiated opinion that alien life is common. We live in a culture that is very different from those in times past in the respect that today we see ourselves as occupants of the universe rather than occupants of Planet Earth. Even 200 years ago most people's metaphysical concerns were with their destination after death: heaven or hell; and these domains figured in their thinking as the only domains other than earth itself (and, from a practical point of view, this was true even of those who pointed their telescopes skywards). As knowledge of astronomy percolated into the realm of general knowledge and (for many people) the simplistic beliefs in heaven and hell retreated our view of our place in the cosmos was transformed. We now know to think of ourselves not as the centre of the universe but as a microscopic agglomeration of carbon molecules in one arm of one galaxy among billions. This has produced a feeling of loneliness that the tales of science fiction, both in literature and in films such as Star Trek, can tap into. In my opinion, the void that religious world views used to occupy has been filled by another fantasy in which we are not alone; and this is the reason why we are prone to believe that there is indeed life beyond our own planet.
A final thought. Suppose it is the case that we are alone in the universe. What does that say for the tussle between theists and atheists? I think it's fairly balanced. The theist's world view is strengthened because humankind really would be as special as their churches tell them. But, for the atheist there is an interesting counterpoint. Your fine-tuning argument argument looks to be in fragments now, does it not?