I've heard people call it the 'flinch factor'. I think most keepers have it to some degree. I flinch when I'm struck at, and I don't fear getting bit at all. I've been keeping snakes for 5 years and I've been bit plenty of times, a couple of pretty bad bleeders but even then it's no big deal. I don't care about getting bit, but I still flinch every time! Drives me nuts.
Actually I have a story. I used to have a rough scaled sand boa. He was a TINY little thing and absolutely nuts. I got bit all the time, but he never broke skin and often I could barely feel it. Sometimes I'd be holding him and not paying attention (checking temps or cleaning or something) and I wouldn't even realize he was biting me until I looked at my hand. No pain involved whatsoever. I never hesitated to scoop him up out of his cage, I had no rational fear of him, but I'd often flinch and get chills down my spine when he bit me.
I think it's an instinctual thing, because it's certainly not a rational or even a conscious fear. I mean, I absolutely adore all of my snakes and I've never been afraid of them but there's some impulse in me that fears them. Or something like that. In any case, I find it extremely fascinating.