Difficult one, most seem to say, when asked why they don't eat 'because i don't want to', or 'don't feel like anything' the real reasons can be quite deep seated, and as we only really get them for 20 mins, its not long enough to open that can of worms, and to be honest may not help even if we did.
I think probably a lot do it due to peer pressure, children of that age live in a different world to us adults usually, remember back, everything seems very different, you think different, act different etc.. peer pressure is a very real thing, it means acceptance into the group (tribe) which is so important to that age group. So maybe the kid that lacks a little self confidence will do anything to get in with the crowd, even if it does the opposite!
Obviously sometimes other forces are at work, such as abuse etc... i would imagine this would be the hardest one to crack, as many of these kids are screaming inside!
I think maybe a lot of it, as always seems the way, is down to attention? we all need attention and love to get through, and without both we start coming off the tracks a little.