As so often with mathematical treatments of complex problems, one of the key benefits of creating the model is that it forces you to make explicit your assumptions.
Rey’s analysis of what makes a relationship sustainably happy is this: relationships will be stable if the effort gap, the difference between the effort you need for stability and c*, the level of effort you enjoy putting in, is tolerable. If it isn’t, you will tend to reduce your level of effort below the level required for stability; this takes you out of the area where you can sustain an acceptable level of x, and into the unstable region. Reduced effort leads to reduced affection, which leads to reduced happiness, which leads to a reduced ability to put in more effort while remaining tolerably happy. As Mr. Micawber says in a different context: Result, misery.
H/T Michael Meadon
Comments