why simple calorie counting doesn't work

The century old scheme for estimating how much energy in food is used by our metabolism is only a crude approximation, but many of us use it as if it was fact.  It can be a useful guide - particularly when you are trying to lose or maintain weight and journal what you eat, but sorting out how your particular body behaves is very complex and an individual task.  A nice post by Rob Dunn appears in SciAm

Fortunatly people have bodies that are good at self regulation.  Unfortunately many of us have managed to break that system by being overweight for a systained period.