[student0]

Why do exothermic reactions require less energy to break bonds and the output of energy is greater than the input energy? And vice versa for endothermic reactions. I am having some trouble understanding the reason behind it.

[mjc123]

Some bond are stronger than others.

[Enthalpy]

The bond strengths at both the reactants and the products vary, so only a numerical comparison tells if the reaction is endo or exothermic.

Why bonds are stronger or weaker isn't obvious. Heavy software computations can give approximate values, but essentially they are measured. Tables:
http://www.chem.ucalgary.ca/courses/350/Carey5th/useful/bonde.html
http://ch301.cm.utexas.edu/section2.php?target=thermo/thermochemistry/enthalpy-bonds.html

Note 1: don't forget the bonds that are not written. Solid C, Al... are full of bonds despite these are not explicit. Reactions only write "Fe".

Note 2: bond energies depend on neighbour atoms too and on the strain in the molecule. This explains the discrepancies between the tables. Many corrections try to include these effects, but they become seriously complicated without achieving an excellent accuracy. So bond energies are conceptually very important, but when accuracy decides, we need better methods.

Note 3: molecules vibrate, rotate, and they may stick to an other to form liquids and solids. All this is energy, which contributes to the heat of a reaction. Less than the bond energies, but important for accuracy.