In some respects, I wonder why I do it too. The trigger was reading a story in the mid-90's by a motorcycling journo who did it. He was hallucinating with pain (forget what sort of bike) and ended up kipping by the roadside and missing the deadline. I thought something like that could be quite a good physical and mental challenge, mentioned it to 2 or 3 mates and by then, it was too late to back out without losing face. Boiling it down, I guess it's stepping outside your comfort zone, just like bungee jumping, white water kayaking or even changing jobs etc - it's stimulating.
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