The lower the HOMO energy, the lower the energy of a molecule than that of its constituent atoms when uncombined, and hence the more exothermic will be the formation of a molecule from its constituent atoms. Of course total energy is always conserved. The real driving force towards stability is the increase in thermal (momentum-space) entropy owing to the exothermic formation of the molecule. The formation of a molecule costs a decrease in configurational or position-space entropy, because originally separate atoms are now forced to move in lockstep. In order for the molecule to be more thermodynamically stable than its uncombined atoms, the formation of the molecule must be sufficiently exothermic that the increase in thermal entropy is larger than the decrease in configurational entropy.
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