bond enthalpies

[Sammy Toscano 1D]

Why is bond formation exothermic?

[Shweta Chawla 2D]

When the "product" molecule is formed, it tends to exist in a more stable state than the initial reactant molecules, thus containing less energy. The difference between the energy of the initial reactant molecules and this final product results in energy being released, leading this process to be exothermic.

[Rachel Lipman]

An endothermic reaction occurs when it takes more energy to break bonds in the reactants than what is released when new bonds for in the products.

An exothermic reaction occurs when more energy is released when new bonds form in products than what is used when bonds in the reactants are broken.

It is simpler if you think of it in terms of the latin roots.
Ex - out (heat is released outwards from bond formation)
En - in (heat is absorbed when bonds are broken)