I was reading a thread on UKclimbing recently where people were talking about the routes they had enjoyed most that year. Unsurprisingly, the majority of these routes were at or just slightly below the maximum grade that each individual was capable of climbing. The excitable and ambitious were perhaps more likely to pick a "best route" nearer their limit. The more mellow (or perhaps falsely modest) keener to pick a slightly easier line.
I've sat opposite climbers who have dedicated their whole lives to the pursuit of hard moves. Sometimes these chaps and chappesses were even going for bold routes to. They grasped for reasons: more interesting moves; better lines; a more finite moment. These are all arguable points for the pursuit of the extreme, but smack a little of justification rather than causation.
It was as I read this thread that it dawned on me that the question of "why push yourself on rock?" is ultimately the same question as "why climb?". Climbing is rarely about simplifying life and it certainly isn't often about taking the easiest line. Unless we're talking pinnacles, the easiest way to the top is usually up a scrubby path. PLATITUDE ALERT: We chose climbing not for the destination, but for the journey. (If I spoke a little more like that all the time maybe I would be sponsored). But, yes, this is true. The reason we took up climbing is because we'd had our fill of walking. The inane monotony could bring a certain kind of hedonism, but this was dull and without focus.
The first time we were drawn to the vertical was the first time we took a different path. Rather than becoming blanded out into where we were heading, we were forced to engage in the acute physical riddle afore. To try and climb a harder line is to be halted more abruptly, to be velcroed to that moment and forced to introspection. At the farthest limits of our capabilities there is the purest parallel to what was our first moment of climbing.