No matter how experienced I get, sometimes I feel like there's no way to wrap my head around the concept "I will be tortured tonight. I better not be late!" I'm both eager and fearful, and I always question my own sanity a little. I think it's because that's the moment when I reflect how I don't have to do any of this. I could walk away. I could even just ask them not to hurt me. Why the hell am I presenting myself for something that will hurt me?
Before I sound too much like a self-punishing neurotic, I should point out that I have this problem on rollercoasters and waterslides too. Now, I love rollercoasters as pure speedy joy and they don't cause me pain or make me sick. But if they stopped it at the top, after that long racha-racha-racha climb and in sight of the sheer hugeness of the first drop, and offered to let me off if I wanted... well, I wouldn't do it, but I imagine the feeling I'd have saying "no" is a whole lot like this one.
(Rollercoasters are really the perfect kink metaphor. You're strapped in, you're scared but trust there will be no real harm, whether you enjoy them is part psychology and part physiology, and they look so different from the outside than they do when you're on it.)