dust eater

at what point does a personality trait become a 'disorder' that can be diagnosed? (just wondering after seeing ODD (oppositional defiant disorder) in the media recently..)
is any personality trait that doesn't let u go through daily life 'comfortably' a 'disorder'? but isn't it 'normal' for people to cope with situations differently?
is it a criteria that the person being diagnosed prefers, or realises after receiving therapy/treatment that they are more comfortable with, the alternative?