The life of a human being is essentially different from that of other people, to a point that sometimes mutual understanding is impossible. Not only empathic understanding, but the most basic linguistic understanding.
If comprehension is not guaranteed between two individuals that are genetically almost identical (that is, any two people), then... what about a non-human intelligence? An extraterrestrial being, having evolved in a completely different way, maybe even with a different body chemistry, might not share our thinking logic.
Emotions, reason, science, art, progress, technology... are all human constructs, concieved in human brains. What can be said about a being that has developed something completely different than a brain? What is the definition of an intelligent life form? A conscious one? One that modifies its environment? One that can feel emotions? One that resolves equations?
By the way, there is a book (and also a movie), "Solaris", by Stanislaw Lem, that discusses this issue. The incompatibility of human intelligence with a peculiar entity: a liquid being covering the whole surface of a planet; a living ocean that has no concept of species, or groups, or communication, because it is a unique individual. Some events hint that the being "thinks", but it is not understood what, why or how. Don't worry, I did not spoil the story for you, there is much more ;)
It is very likely that we are not alone in the Universe, but, will we ever be able to talk with someone if we ever find them? Are we surrounded by life, but forever alone?
