Personally, I believe it's the height of human arrogance to believe that Earth is the only planet the contains intelligent life. Of course, if we are, that would make us the most intelligent species in the Universe, so why wouldn't they want to cling to that idea? It's merely an extension of the same arrogance that leads some to believe that their race, gender, religion or whatever else is superior to all others. I think they have taken Shakespeare's quote too literally.
Hamlet: Act 2, Scene 2: "What a piece of work is man! How noble in reason, how infinite in faculty! In form and moving how express and admirable! In action how like an angel! In apprehension how like a god! The beauty of the world! The paragon of animals!"