why do some reactions release more energy than they absorb , while others do not?

Answer 1

The simple answer, is that all substances have energy stored in their bonds.

If the energy of the new products in a reaction is less than the energy that was stored in the old reactants, then the extra energy is released

If the energy of the products is higher than what was in the reactants, then the reaction must absorb the extra energy from the surroundings.

Answer 2

Consider a reaction like you would a mere phase change.
Melting
solid + energy => liquid
Energy is required to make the ordered particles of the solid state become liquid. Because there is more order in the solid state, it requires less energy to maintain (i.e. lower temperature). However, the excess energy of the system travels to the solid compund until it melts (melting point or, in a reaction, activation energy).
Freezing
liquid => solid + energy
The liquid cools back down to the solid state- it gives off the energy it has received. Like before, the point at where the particles no longer have the minimum energy required to be liquid and start to form the lower energy state of solid is the freezing point (or, in a reaction, activation energy).
The point is, some reactions take low-energy states and add energy (an endothermic process), and some take a high-energy state and remove heat (an exothermic process).