Self sabotage. Most runners are victims of it, at least most runners I know.
I was doing a long run with a good friend a couple of weeks ago. I knew something was off because normally she would hammer me through 90 minutes at a hellish pace and then go on to run another 30-45 minutes, but this particular day she could barely keep up. She was off...some kind of flu or bug...so when we came around our 90 minute loop I said she should stop. No NO NO. Must go on another 30 minutes. She preceded to get sicker until finally she was forced to take a couple of days of nothing and then, poof, she was better.
It's so easy to see this madness in someone else. You see the desire to be better, to go harder, to push the body beyond it's capacity for recovery...and the body rebels....and the person takes 2 weeks to recover rather than 1 day.
I'm not innocent of this crime. Just last Friday my coach stopped me mid workout because I was clearly sick. I couldn't even cool down, yet I kept asking, "can I do the workout tomorrow?" "No". And of course what did I end up doing, I ran a 20 minutes tempo run set on max speed on my treadmill.
Maybe we runners like pushing the body to the edge. Maybe we want to punish ourselves for our failures. Maybe we're just plain nuts. In the end, we must remember that we will always be better off day to day when we listen to what the body is screaming at us.