Good grief, thanks for that image :)
I wouldn't be physically fit enough for all that at all, I don't think. But I can see you'd need to be mentally fit as well, you'd definitely need to be able to keep a lid on it when you're inching along with your face pressed to the rock. I guess if you're confident in your physical ability and bear in mind that other people have been through there before, there's less reason to freak out and get stuck?

Maybe that's the difference slightly, that most cavers are going in places that have been previously explored, you kind of know where you're going and theoretically what to expect. But with Howburn Digger's excellent anecdote above, what strikes me is the element of the unknown. Maybe it's easier for human beings to damp down the bit of your brain that deals with 'aargh I'm squeezing through a tight gap in some rock' than it is 'what ghastly things could be lurking in the dark round the corner'. The first being a bit more rational (don't ask me to do it mind) and the second being something wrapped up in instinct and superstition. Your horrid sounding s-bend sounds like it contains both though, ugh.

God I'm such a coward. But I might crawl through some stuff to see cave paintings of animals in the light of a flickering torch. That might motivate me :)
But you must have seen some cool formations down there. It must give you a different slant on the daylight world, when you know what's down there, unseen, unthought-about by the people scurrying about on the surface. I still think it's a freaky thought. In amongst the solidness of the earth, squeezing about in little gaps. (pulls face involuntarily)