I have had this conversation with my father who has been riding road bikes for 40 years or so. When I was growing up, he was always preaching about the importance of a helmet when riding on or offroad, but somewhere along the line, he stopped wearing one on his road bike.
When I asked him about it a few years ago, he almost looked ashamed at his decision to ride without one and couldn't give a reason for his decision. He told me that he new he should wear it, but just enjoys it more without it.
He owns a bike shop, and we had this conversation a few days after one of his employees was hit riding home after work at 30MPH. He was not wearing a helmet and the only injury he sustained was head trauma.
To this day, he doesn't wear a helmet. I do not believe I will ever understand this.