The reason(I'm optimistic aboutfinding aliens) I think is because as humans, we want more, want there to be more, want answers. Answers to questions like why are we here? If other life forms exist then these questions get closer and closer to answers. An example might we be here to learn about other lifeforms, or to teach them or to do something in relation to them. Also Gods plan(or whoever he is) surely must have included them too in his strategies. How does this affect the potentials answers to our human unanswered questions?
They would change how we think about things. This is what we crave. We want more answers. Also I think the human life-form is lonely and yearns for company. Even at the individual level, personally I feel that we're a bit isolated in the universe and feel that exploration, variety etc...more. To think that life exists only on this planet is profound. We'd be incredibly lucky. Though, many of our questions that attempt to reach outer worldly ideas such as the reasons for earth, our lives and the goals of individuals and our species in general, would remain out of reach. Perhaps without further knowledge of that which is out of our reach, we can not know the answers we seek in this regard. Funny, in a world that we live in, there is so much already. We have entire communities, we have rap music, gyms, electronics, laughter, sadness, trees, Gravity and other magnificently complex ideas. Psychology, programming, pregnancy and birth etc... With all this you wonder why we need more? But perhaps it's because of all these we want to know what it's for or why it's so magnificent or why it's here, why we experience the things that we do. Perhaps this is why we want more.