"What was it about being locked up together that makes love wane? Is it that ordinary day-to-day life is not sexy? Is it they found out they had little in common?"

I think not. I think one of the things that keeps a marriage going is that when we go out in the world, we experience much that is uncomfortable, frustrating, and such like. The world beats us up so much that when we return home, to be welcomed and comforted by those we love, it renews our love. Home and our loves are the places we go to at the end of the day.

That daily spark of renewal has been lost and I think it might be the loss of it that has cooled the fires as much, if not more, than if the spark dies differently and home is no longer the place of comfort and love.