|Reaction coordinate showing two possible directions of spontaneous reactions; the region in red depicting an exergonic reaction, quantified by a negative change (ΔG < 0) in the value of the Gibbs free energy of the reaction of products on going to reactants. |
In 2001, David Hwang, in his article "The Thermodynamics of Love", stated, in the context of a typical mate selection human combination reaction, such as in the process of two people falling in love or the process of a full human reproduction reaction, that: 
“If the male-female reaction has a negative free energy, then it is expected that the system will spend most of its time together as a strong couple.”
The principle is an extension of spontaneity criterion applied to human chemical reactions, in short. 
1. Thims, Libb. (2007). Human Chemistry (Volume One) (Hwang free energy principle, pg. 117; graph, pg. 113). Morrisville, NC: LuLu.
2. Hwang, David. (2001). "The Thermodynamics of Love" (PDF), Journal of Hybrid Vigor, Issue 1, Emory University.