Aside from technical issues it can be difficult, at least for me. Unless unexplained and I can learn something from them I trash my negatives that are no good for technical reasons. Otherwise, I save them all. As I stated in the other thread, my ideas about an image can change when stripped of my initial expectations and become imbued with the filter of time.
And sometimes I think I just need to see them in the positive before passing judgement... A good reason to proof all your work, something I don't do but probably should.