I extrapolate from Earth outward.
There’s hardly a space on Earth that isn’t filled with Life. In recent decades we’ve even found that forms of life can exist in environments we previously thought utterly inhospitable, super hot, super cold, etc.
It just seems to me that there has to be life out there; and that it’s likely to have developed to be intelligent in some places. There’s just too much space, too many stars, too many planets.
And believing in God, the idea that only this one planet has life seems to me a completely nonsensical limitation on a God proven to possess such astoundingly unlimited creativity.
On this question I like the Fermi Paradox. Also, God can do whatever he pleases and wherever he pleases he can create life or not and keep it exclusive to earth. There is just no hard evidence of life anywhere else in the universe that we can explore. Just lots of conjecture and probability.