When one wouldn't let themselves fall below a certain line. What that line is depends on the person. It CAN be used as an excuse, but for many it is a way of life.
I'm an athlete, and if I want to be the best, I must reach and maintain a high standard.
This is an excuse used by people who are afraid to get into relationships. Since no one knows what these standards are, no one can possibly be expected to meet them. Therefore, all potential love interests are disqualified before the first date.