Think of the coolest married couple you know.
Now make one of them your best friend of a quarter century.
Now have your best friend come tell you that he doesn't know why his marriage is suddenly at the brink of divorce.
There is no answer. There's no "reason," in the traditional sense.
There's no advice to give.
These two wonderful people are together so much greater than the sum of their wholes. Yet here they are, and they don't know why. I sure as hell don't.