The simple, chemist’s version of the explanation is that a bond will form if it represents a more “stable” configuration of the atoms involved than their unbonded state. This necessarily involves the descent of electrons into “potential energy wells,” such that some chemical potential, or “free energy”, is irreversibly dissipated during the process of chemical bond formation. Free energy, or Gibbs free energy in honor of the dude, is a state function which takes into account both the energy/entropy changes inside a system and the energy/entropy changes outside the system, thus making it the suitable “potential” for determining the overall outcome of a process. The most stable configuration of a molecule, according to the statistical thermodynamics of Gibbs, is simply the one with the lowest free energy, thus revealing the concept of stability to be a property of resistance to (fundamentally quantum) statistical fluctuations. Chemists refer to free energy as the “criterion of spontaneity” because the free energy change for a process dictates whether or not it will occur spontaneously, meaning without the input of external energy. So what I just told you is that the reason chemistry happens is because the universe is spontaneously dissipating free energy. But why on earth is it doing that?