This is not hard to understand. We (most people) want to believe what we are told, we think the best of people. When your friend gets cheated on but takes the girl/guy back you think "What the hell" but he/she thinks "They love me, they said they are sorry, they deserve another chance"
This is pretty much normal behavior. We want to believe what we want to believe.
So back with JFK...when he was smiling and cracking jokes...everyone loved him...even women...didn't matter that basically everyone knew him and his bro were doing an Eiffel tower on Marilyn. They wanted to believe in him and what he said.