Why would anyone run further than a marathon?

"Where next? Oh, over there I reckon. Right lets go. Yup, it's this way. Left hamstring is feeling tight, need to keep an eye on that. Need to drink more water. Not hungry, must eat though. Drink more water. Is that a hotspot on my foot going to turn into a blister. Drink more water. Where is the next stop. Ah, round this corner. Excellent. Stop for a bit. 60 kilometres covered. Been running for over 6 hours, plenty of distance left to cover though..."
That was a short section of the 100km course I ran last summer and the entire distance was covered with my brain asking these questions all the time. Someone asked me during this run what I was looking forward to most at the finish. A beer perhaps? A swim in the river? Simply to stop running? Actually the answer was none of the above. I wasn't looking forward to the finish, I was simply looking forward to the next step. The simple act of putting one foot in front of each other. Getting to the next fence. Running over the next hill. The journey was the goal, not the finish line.
Ultra running is different. Not only because of the obvious difference in the length of the events but in the manner of running them. Line up at the start of a 10km race and you know you're going to push yourself hard to cover the distance in the shortest time possible. If you've run a few before then you'll have your personal best and you'll be looking to run faster than that. If you don't you'll be making excuses to people that it was a hilly course, or you hadn't trained enough, or it just wasn't your day. Marathons are pretty much the same experience. Tell someone you've run a marathon and invariably the first question is "How long did it take you?" Unless you ran a personal best then you'll be making excuses why you didn't.
Imagine two runners comparing their finishing times at different US events. One finished in 15 hours 7 mins, the other in 23 hours 23 mins. Who has the bragging rights to claim they are the better runner? What do you compare these times to when deciding whether these are good times or not? For the marathon (or shorter distance races) most people know what a good time is. That's what I love about ultras. You run for the enjoyment in the knowledge that the finishing time is pretty much irrelevent. Those finishing times mentioned about are actually both course records at the Western States 100 (18,000 feet of elevation gain and the Hardrock 100 (33,000 feet of elevation gain. Both are outstanding times run by elite runners.
So why would anyone run further than a marathon?
Everyone ultra runner will have a different answer but in my case it's simply because I love to run and I don't want to be held back or be judged by traditional race distances. A race of an hour just couldn't scratch the itch I have. I love to be out there running over the hills and travelling great distances using only my own two (often barefoot) feet.