Why do I need to eat it all in one go. No one else in the family is going to eat it all if I leave it in the cupboard.
Maybe because it's there...and it tastes good? It usually disolves quickly and it's easy to get through a lot in a short amount of time without feeling satisfied. Maybe
I know that I would eat what was around me as long as I liked it. On reflection, I found that this didn't just apply to the 'bad' foods. Put a bag of grapes next to me whilst I'm watching the TV, and I'll happily work my way through the whole bag. Same goes for nuts, but they take a bit more chewing and so I'm more aware of what I'm doing.
Chocolate though, can just disappear
Think I might be over simplifying things here though