Well for starters the 1960’s was a very tumultuous decade, civil rights, the Cold War, Vietnam, the space race, Rock music, drugs... To view it as Americans did at the time you’ve got to realize everyone thought that Russia and communism was an existential threat, and his assassination was only shortly after the second red scare and the Cuban middle crisis the closest we’ve been to nuclear conflict. When a nation is locked in a life or death struggle with its arch enemy losing their president is quite a traumatic experience. Added to that Kennedy was viewed by many as a man who could bring change to the nation, he had put work into the civil rights movement which his brother furthered, he began the race to the moon which cemented American dominance in space. His assassination was one of the first in our modern age, one that was televised for the whole world to see, that had to play a major role in the zeitgeist of the era (seeing your presidents head explode on TV was definitely a first). On top of all that his brother Robert was killed as well, though years later it surely affected many. Finally one of the biggest influences is the conspiracy theories that have cropped up in the wake of his death. The murder was the most publicized case in human history and when you have millions of people knit picking the finer points it leads some to think that there were mistakes made in the investigation, maybe even a cover up, but I doubt we’ll ever know the whole story.