Unfortunately, I do think that a lot of people get into very long distance running for the wrong reasons, and not because they really enjoy hours and hours of training.
- To be extremely fit - Large volumes of low intensity exercise do not necessarily make you "fitter". There is no single standard for "fitness", and you are only fit for what you train to do. After all, Usain Bolt is quite fit. He trains to be sprinter, and he is fit for sprinting.
- Obsessiveness - I see a lot of people who become obsessive about their training. If you're training when sick, injured or tired in order to meet some magic quota of miles, then that's a good sign that you have an unhealthy relationship with running. I've known too many runners who are no longer able to run because they didn't have enough sense to listen to their bodies and rest when they needed it. If you truly love running, run in way the will keep you healthy and able to run for many years.
- Something to prove - Over-achievers, you gotta love 'em. I know a lot very successful people who feel that they must be successful at everything, and they must do it by working harder and longer than everyone else. It really amounts to big numbers for big egos.