The microcosm that is Earth first appeared to humans as the center of everything. The babe, in his mother's arms, proceeds on the same assumption. She is there for his needs, solely. We hesitate to broaden our perceptions because this minimizes the self. Mother does not exist solely for me, therefore I may go unattended. Father chats with the other children, therefore I have not captivated his attentions and may go unnoticed. And in like manner we follow the child into adulthood to discover the human species grappling with the concept of not being alone, the one and only. There are other intelligent species in the Universe, therefore I am not God's crowning achievement. They can levitate and zip about and disappear, therefore I am a technological dummy. All very deflating, especially for the pompous. The reaction is reluctance, active denial, or a pout - non-participation. Look right at the facts and fail to process them. Process them and refuse to accept the conclusion. Develop countervailing theories and attempt to substantiate them. Close all the doors and windows and develop an avid interest in a hobby. Just don't have time for all that nonsense. Well, of course, it won't go away.
Humans, adjusting to the concept that they are not alone, go through stages not unlike the small child adjusting to nursery school. They feel, ordinarily, that they are the masters of their small Universe. A cry of distress brings Mommy or an older sibling running. Throw out a beaming smile and an adult starts chirping. Works every time. Then comes the shock. The babe is not alone. The first reaction to the sight of a room full of competitors and a distracted Mommy substitute is to cling to Mommy. Humans, reacting to the alien presence, pray fervently. Save me! The babe may strike out at competitors, bonking another playmate, or snatch toys away for a personal horde. The alien presence almost invariably brings a cry from some angry human - leave us alone, the Earth is ours! Unable to mold the nursery to one's liking, the babe may next pout. Sit in the corner, lower lip out, glowering. Want a cookie? No! Perhaps non-participation will make it all go away. Some babes pout forever, and make it a lifelong practice. We find these humans turning their backs on evidence of the alien presence. Non-participants are the least likely to turn and become actively involved - curious and exploratory. In fact, the vocal skeptic is most likely to suddenly stop protesting and let it all sink in. Look to the history of avid believers and one will often find a skeptic. The skeptic gives birth to the advocate.