Chapter 12 says true love does not evolve or revolutionize. The instability appears not in love itself but in people who fail to protect and remain faithful to it.
That is a helpful distinction because it resists romanticizing drift. Changing feelings may be real, but they do not prove that covenantal love was meant to be disposable.
The note also works beyond marriage. Every durable love requires practices of protection, not just the memory of an initial feeling.