Jim Hines gets to the heart of the matter in under 4 minutes:
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Don't just do something, stand there! Reflections on the counterintuitive behavior of complex systems, seen through the eyes of System Dynamics, Systems Thinking and simulation.
I took a stab at making the model shown in this video. I had to make a few guesses about functional forms (most notably the table function defining ‘Love Effect on Desired Time Together’) which the model is more sensitive to than I would like, but this recreates the cyclic behavior seen in the vido and can also recreate a few other interesting behaviors (settling, absence-makes-the-heart-grow-fonder, and passionate swings):
https://jpaine.mit.edu/sites/default/files/documents/Paine_Hines_Romance_model.mdl