“It is not your love that sustains the marriage, but from now on, the marriage that sustains your love.” – Dietrich Bonhoeffer
My three favorite author/theologian/teachers expound this idea in this video.
Piper, Carson, and Keller on Sustaining the Covenant of Marital Love from The Gospel Coalition on Vimeo.
John Piper has written one of the most helpful books on marriage I have ever read. Grab This Momentary Marriage at WTSBooks or get a free PDF from the Desiring God Web site. I also recommend his sermon series on marriage it is based on.