The reaction to such events and encounters is engrained in the current and pass experiences of the individual, and the groups who witness such things, but there is also social conditioning basis upon the nature of the world and the creatures that occupy that world.
The nature of the human species in all its aspects is perhaps one of the fear factors that causes the reactions of those individuals and groups to the various situations involving encounters with UFOs and alien visitors. The fear factor can be real, such has those related to personal experiences of a domestic nature, such has crime related, social unrest, unknown aspects of what they are encountering and the intent of the unknowns.
Domestic affairs such has the intent of the human species to one another is a cause of some of the fear that people have, such has those that commit murder, burglary, torture, injury to another and so forth. The input of the media who report such darker elements of the human species intent is also another ingredient which adds to the reaction of the observer to such strange unknown events and experiences related to UFOs and alien encounters. The nature of entertainment such has films related to real events and those that are fiction cause psychological reactions and possibly leaves permanent memories of such emotional content, which maybe recalled and add to the fear when some encounter bizarre unknown occurrences in their lives, and more so if it transcends beyond their normal experience or repetitious nature of their experiences.
So our lives and the nature of the world in all its negative aspects is adding fuel to the fire, the wars, the crime, the constant input of negative news, human nature in all its darker aspects are all parts of the larger fear that is a cause of many reactions to unknown quantities, since the many like to control their lives to avoid the negative aspects to domestic encounters which could harm them, but has we know UFOs and so called alien encounters sometime do not abide by our rules.