I've been a life-long shark lover since I was a young kid. While other kids got sent to tennis camp, horse camp, etc in the summer, my parents sent me to shark camp. Was one of the greatest experiences of my life and started a life-long passion.
I've been diving for 20+ years and I've seen many sharks during this time. Never once was I afraid of the sharks. I'm always the nut who wants to pet them.
I experienced one of the thrills of my life last fall with a trip to Guadalupe Island to cage dive with great white sharks. Typical of my past, I spent the ride out trying to figure out how I was going to touch the sharks. Once we were anchored and setup, I was the first diver in the water and the first shark showed up within 5 minutes.
Tying this back to the original question, why the fear in great white sharks, I can tell you from experience, my first thought was "holy crap" and I couldn't get my hands back in the cage fast enough. Although I knew I was safe in the cage, any thought of reaching out to "pet" one or swim outside the cage was removed forever. There was zero doubt that any venture outside the cage would be my last.
Why the new found respect. They were huge (I was pleased with myself that the first shark I saw was a world record 22' female that was disappointingly later pointed out to be a 13' male). The could be very fast. Lastly, even in the cages, you always had the feeling they were hunting you. Checking you out from all angles, trying to surge from below, ... That was a new sensation for me and I prefer my place in life to be the top of the food chain.
I love sharks. I love great white sharks. I will dive with every species of shark outside of a cage except for great white sharks. I do reserve the right to change my mind about free-diving with gws, but I already have enough cool scars for right now.