Most people put forward what I call the "A.L.I.E.N." hypothesis or "Argument for Life by Invoking Extreme Numbers". What this argument basically says is that if you imagine the number of planets there could be around each star, the number of stars in each galaxy, and the number of galaxies in the universe you get a very big number and because the number is big, this makes life likely.
But this is not necessarily a valid logical argument. Let's suppose there are a billion planets out there capable of supporting life. That's a lot. But if the probability of life starting on those planets is one in a billion, only one is likely to have life.
And this is the problem. We do not know what the probability of having the right materials on a planet for life is. We don't know the probability that those materials on a planet will form self replicating molecules. And we don't know the probability that single celled life may develop on that planet. Such things might require lots of steps that are each mildly improbable. For example, do you need a sizable Moon? How important are tides to the formation of life? Are underwater vents essential ... and so on.
So it could be the case these probabilities are so low that, even with the large number of possible planets, we'd only expect one planet to have life. That's why looking for at least one other instance of life within our solar system is vitally important. It gives us a major pointer that the Earth isn't a freak.