The strongest line of argumentation you get from those who believe aliens exist is "Well, they have to exist. Earth can't possibly be the only planet with life." Basically, because it's hard to believe means it can't be true. Not the most solid rationale. A non-answer.
There's no substantive answer to the question "Why can't earth be the only planet with life?" The best that can be offered up is "Well, it would be arrogant to think that." Again, that's not a rationale or justification. For one, whether one ascribes to evolution or creation, humans had nothing to do with the process, so how is it arrogant to think such? For another, just saying it's arrogant doesn't actually answer the question. In any way. In actuality, the opposite is true: it would be the most humbling thing ever to know that, whether by creation or evolution, I ended up on the one habitable planet in the universe.
So, yeah, the full extent of the pro-alien argument is: life can't possibly exist just on earth and it would be arrogant to think that. You'll have to excuse for my not being swayed by that line of non-logic.
Not adamant in my answer enough to directly contradict you. But the logic I've seen behind the " we cant possibly being alone" idea was the sheer math behind the number of stars with the number of planets spread across the multitude of galaxies.
The sheer massive volume of potential life bearing planets would lead one to think that the Goldie locks conditions are repeated many times in the universe.
Now, one would have to believe the mechanism for life (divine intervention or evolution) would work the same throughout the universe.
So, if God really did create only us, the number of habitable planets means nothing. If we evolved on our own, then one could reasonably think other life on other planets did as well.