dy/dx of cool

Nothing is ever equal. It just doesn’t work that way. It’s also true that everything changes. I had a realization a few months ago, that most of my friends have blindingly cool spouses, who over time seem to get cooler. There are exceptions to every rule but I give you this chart as gospel truth.

I’m pretty sure nobody will dispute it’s veracity at my house.
Does anyone know what causes this phenomenon? I can think of a small handful of counter-examples, but they may be errors in judgement.