I like Bruce's take on "Why do them"? I really can relate to those that don't have the time, don't have the horse, and don't think the fun factor is high enough. There are horses and people that physically can not do a 100.
I think one of the most overlooked aspects of a 100 is that you really have to have the desire to do one. Having the right frame of mind is 90% of the battle. I am 16 years older then when I did my first 100 in 1990, I am at least 25+ lbs heavier, I am much less fit ( I was doing ultras until 1995 and now am lucky to be able to complete a short course at a R&T).
So what is different today that makes me really like doing 100s (meaning the fun factor as well as phyically hurting) better then 1990s. What makes me more successful now? It's definitely not because I am in a better place because of my age, physical fitness, body conditioning, etc. It's not because I have more time to train and condition - I definitely have less. In fact, I go to less rides now then 10 years ago. My 100 mile horses don't have any more talent then the horses that I was conditioning in the 90s. The thing that seems to be in a better place is my mind. I think having the right mind set and a good attitude can make all the difference in the world on whether 100s are high on the "fun" meter.
Two weeks before the 20 MT 100 I went on a 20+ mile training ride and for the life of me could not imagine having to go 5 times further in two weeks. I finished my training ride at 4:00 and couldn't imagine being in the saddle until after midnight. I went to bed that night thinking I still wouldn't be finished riding the 100. Yet, on ride day, the 100 really did seem like a piece of cake. It never onced crossed my mind that it was too hard, it was too long, it was too cold or too dark. As whimpy as I am, it never crossed my mind not to start because of cold or possible bad weather. Everyone who knows me knows that I don't do rain, I don't do sleet. I don't have to :) The one nice thing about where I live (so cal) is that I don't have to ride in bad weather.....so I don't. But because of my desire and mind set I overcame my normal whimpy tendencies about weather.
Sometime our attitude and mindset is the least used of tools available to us. The only thing that changed in the two weeks before my last big training ride and the 100 mile ride itself was my attitude and mindset. I certainly wasn't any more prepared for the 100 two weeks after that 20 mile training ride. The only real thing that has changed since the early 90s and today that has made me a successful 100 mile rider is my mindset and attitude. My physical attributes were much more conducive in the 1990s to success at the 100 mile distance then what I have to work with today. Attitude is what can make the difference between a successful ride and an unsuccessful ride a good ride and a great ride....
