Hard training has a way of instilling a "They're not one of us" attitude in troops. On my own BMQ, SQ, and BIQ I remember guys who quit being alienated, taunted, and generally treated like outcasts...
The troops on grad parade are a proud bunch. They've accomplished something difficult - something that not everyone can do. Its a feeling of superiority - you are Darwin's fittest, the strongest, and most determined. You survived what others gave up on. Its part of the necessary conditioning a soldier goes through in order to have a "never give up" mentality in a time of war. Its stress inoculation, knowing that "I'm a hard motherfucker, and I've been through worse." But with that comes the knowledge that "That guy couldn't hack it". Troops have a way of feeding off those who quit. The more guys who don't make it the more exclusive the successful group becomes. It's the reason for elitist attitudes and resentment towards those who hold them. It's an unfortunate side effect of what I believe to be a necessary mentality.
I'm not taking sides on this issue or justifying one way or the other. Just thinking out loud...