Although this was marketed to us as a book with a Christmas world view, it really wasn't.
His premise is, now that the kids are gone, you can focus on yourselves again. However, there was barely any mention of the idea of outward service. How about: Now that the kids are gone, we can use our additional time and energy to serve others.
I also missed any reference to really asking God, "Now what?" Surely God has an opinion of how we should spend our time and energy. Becoming empty nest-ers doesn't override his Lordship. In what way does He want us to work with Him?
Yes, marriage takes effort, and that doesn't change as you go through life's various stages. But focusing on one another shouldn't be the end purpose. Rather, two become one in order to love God and be useful in His Kingdom.