In a culture where people have learned to accept things like racism, classism, anti-Semitism, and gynecide, it takes an event, maybe a book, movie, or personal experience to change the way people think.
That event for me came when I was about 8 or 9 years old living in Mississippi in the late 40s. It was a late Saturday afternoon in summer. I was coming home on the bus after going to a double feature with some friends. I was sitting at the very front of the bus and it stopped to pick up an older black man wearing a suit. He paid the driver and sat down right across from me. The driver turned and said boy get to the back of the bus or something like that. The old black man just sat there. The driver told him again and the old man just sat there. The driver grabs the old guy and literally throws him off the bus. I look around and he's laying in the street on his back not moving. The driver turned around and said something about damn N**** not knowing his place. I dismissed it at the time but in years to come I thought about that event a lot and how wrong it was. Never said anything about it to my parents or friends but it changed the way I thought about the treatment of black people. Before then I don't think I ever thought about how black people were being treated. They were just there to pickup garbage, clean the floor, sweep the streets, clean the houses, and do all kinds jobs that white people didn't do.