Camera and Pillow, they both tie as winners and I would not pick one over the other as both are just as important. Just imagine in, no pillow, not comfy after a great days hike. No cam, cant take pix of the awesome scenery.
I can't think of any single item that would cause me not to camp. Pretty much anything I would forget could be foraged, improvised, or otherwise done without.
I am not really good at foraging or improvising. I suck so I would rather make sure I have the things with me and not panic because I do not have it with me.