This Saturday I'm attempting my first ever ultramarathon; 34 miles (55km) across the mountains of southern Iceland*. Since I signed up and started training for the event back in January, I've been asked countless times why I would choose to do such a thing.
It's going to be painful. It's hard enough running a normal, 26.2-mile marathon on flat roads, but in this event there is a total vertical ascent of around 2000 metres, and a significant amount of the trail is covered in snow (there is more snow than usual this year, but the organisers have assured us that it is "passable"). The conditions in Iceland are also highly variable, and it's quite normal to experience four seasons' worth of weather in a single day. So I'm fully expecting at some point to be trudging uphill headlong into a gale-force wind with freezing rain, knowing that there are several hours of running still to be done.
So why on Earth would I voluntarily put myself in such an uncomfortable situation? Well one reason is to experience scenery like this...

... but then I could hike the trail over three or four days and see the same things. The real reason is that the discomfort is the whole point. It's a way of stepping outside the mundane and reaching physical and mental extremes that you don't experience in everyday life.
Although I've been training for the race for more than 6 months, I don't know whether I will succeed or will have to be hauled, broken, off the mountain. There's an excitement though to stepping beyond the limit of what I know I can do. The most rewarding experiences in life are never the most comfortable, and the temporary pain only serves to amplify the long-term satisfaction, the joy, of overcoming a challenge. Even if I fail, I'll do so having given my best effort and knowing that I took on something most people would never have the courage to try.
That said, this is just a post-rationalisation and there really is no logical reason—once I got the idea in my head to do an ultramarathon, it wouldn't go away until I tried.
*This is a baby ultramarathon compared to some events—there are events covering 100 miles or more in a single day, and then there are multi-day events like the Marathon Des Sables; a gruelling 6-day run across 156 miles of the Sahara desert. Perhaps the worst though is the infamous Badwater ultra, which starts in Death Valley and climbs 13,000 ft over 135 miles. Competitors have to run on the white line on the road through Death Valley to stop their shoes melting.
Update (20th July 2015, 2 days after the race)
I finished in 8:30:40, much slower than I hoped, but I was limping for about 10km after hurting my knee on a downhill section. This was after the last checkpoint where I could have dropped out, so I had no option but to keep going. Fortunately, a group of hikers gave me some ibuprofen which took effect about 3km from the end, so was able to run the last section and put in a sprint finish. Now looking for the next one to take on...