I have had this question on my mind for sometime. It was partially answered when I met some really interesting people during the Bangalore ultra. There was no one common answer, everyone had their own reason for doing the ultra, maybe because most of them were doing their first one.
All of us know that human body is not designed for long distance running, which usually means marathons, and that makes ultra quite unhealthy (does not sound very pleasant). But the buzz you get by doing it can't be achieved by any synthetic stimulant, and it truly amplifies the power of the mind (the longer the distance, more critical is the mental effort), training primarily helps you only in the recovery process post run.
The reasons were varied - Wanting to quit another addiction - like smoking, constant hunger for tougher challenges, if its there it needs to be conquered, wanting to push limits of physical endurance (almost everyone uses this, but the motives are different), training ground for tougher ultras, my friends (in running club) are doing it (now that is some super positive peer pressure), it goes on....
What will keep them going, and doing more challenging ultras - longer ultras, 24hr ultra, 100mile ultra, multi-stage ultras.... the list goes on. Maybe those who keep pushing the limits or constantly crave for continuous supply of ultras would be a more homogeneous group.