I often struggle with this question. It is difficult to nail down exactly why I love to run so long. I usually don't know why and have practically given up trying to answer that for others.
Nevertheless, I was struck on Sunday by one reason that I run: so I can eat more. Yep. When I'm not running, I get pudgy and eating is not very enjoyable. After running, I'm trim and can't seem to get full.
So, there is one concrete reason why I ultra run. I'm glad that it is such a morally lofty reason. :)