I believe its a deeply philosophical question, one that concerning Theology and the other concerning the philosophy of science : the ability for the process of scientific inquiry to yield accurate predictions, once a suitable theory is proposed and proven.
Theological issues involve mainly relationship between extraterestrial life and God: do they have souls and related questions.
But the relationship between the philosophy of science and extraterrestrial life is even more interesting: we assume the laws of physics are the same througout the universe. What about the laws of biology? The current problem seems to be that there has been no known theory of the evolution of life, but at the same time such a theory is the holy grail of evolutionary biology. Assuming then such a theory will one day exist, one will be able to predict the evolution of life in the same way Newton and Kepler helped us predict the orbit of planets with enough precision to land space probes on Mars for example. Once the mechanism is known, then it becomes easier to predict the outcome of life on other worlds the same way we can predict the existence of volcanoes and dust storms given the chemical composition of the atmosphere of other planets and the physics of climate modelling.
There is, I believe, a great fear associated with the existence of a few bacteria outside of this Earth, and this fear is amenable to study by another science, the science of psychology.
An interesting article here adresses this question:
This would explain the absence of observable aliens.
Because if the rise of intelligent life on any one planet is
sufficiently improbable, then it follows that we are most likely the
only such civilization in our galaxy or even in the entire observable
universe. (The observable universe contains approximately 1022
stars. The universe might well extend infinitely far beyond part that
is observable by us, and may contain infinitely many stars. If so,
then it is virtually certain that there exists an infinite number of
intelligent extraterrestrial species, no matter how improbable their
evolution on any given planet. However, cosmological theory implies
that, due to the expansion of the universe, any life outside the
observable universe is and will forever remain causally disconnected
from us: it can never visit us, communicate with us, or be seen by us
or our descendants.)