It's because of the mass (and energy obviously) defecit between the products and the reactants.
The curve of nuclear binding energy per nucleon starts at 0 (obviously), quickly increases to it's peak at Iron, then gradually drops away.
You get energy released from a reaction which results in an increase of NBE/Nucleon, which is why fusion of light elements releases energy, and so does fission of heavy elements.
This would be easier to explain if someone drew me/found a copy of the curve.
ps. This curve also explains why the transironic (now there's a silly name, I can't remember if it's actually used or not) elements aren't created in your average star, but by supernovae.