I've been riding a lot of years, accumulating more than 10k miles per year for all that time (sometimes a good bit more). I also do somewhere between 15-40 centuries per year, some organized, some JRA, and quite a few 7-10 days of 100+ miles per day.
OK, with that as background, I've never felt inclined to go further. A number of my riding buddies have done multiple brevets (PBP, Cascade 1200, BMB, Rocky Mountain 1200, etc) and I take a fair amount of good natured abuse about just being a "century rider". (Not to hijack my own thread but, to paraphrase the late George Carlin: "Everybody who rides less than me is a wimp, everybody that rides more than me is crazy").
Every year after finishing one of the 700+ mile weeks, I think "I'm in pretty good shape, I should go do a double/brevet/something longer". For whatever reason, I never have. I've had the conditioning, just not the inclination.
I'm not looking for psychoanalysis here, I'm just curious for those who have gone on to ride longer distances what was it that motivated you to do it. Additional challenge? Didn't get enough? Social reasons (friends, significant others, etc)?