I've been asked many times "why run ultras?" and spent many hours thinking about the answer, especially towards the end of long races. There is no good answer, but here are a selection of possibilities http://fellrnr.com/wiki/Why_Run_Ultras
When I first started to write about the "why" of ultrarunning, I thought that would be two or three possible answers, but over time the list has grown far longer. I've had lots of input and suggestions from many runners, and I welcome further suggestions.