Unresolved conflict. I think the most deadly fox, however, is apathy. If you truly are committed to making your marriage last for a lifetime, and enjoying the type of oneness God intends in a relationship, you need to make a choice to keep romance in your marriage.
If you want to put the spark of romance back into your marriage, I have two tips. First, become a student of your mate. Do you know that men and women view romance through different lenses? To confirm this, the next time you are in a Bible study or Sunday School class, divide the men and women into separate groups and ask them to answer the same question: What is something romantic that you would like your mate to do for you?
I'll guarantee you that the men's list will focus on physical intimacy: "Dress up in a sexy negligee" or "Meet me at the front door without any clothes on." The women, however, will say things like, "Take me to a romantic, candle-lit restaurant," "Spend time talking with me" or "Sit in front of a fire and cuddle." Men are motivated by sight and touch, while women want to develop a relationship.
Second, take the time to plan some creative romance. Do something different, out of the ordinary, something that will capture your mate's attention. Perhaps you need to take a weekend away, just the two of you. You'd be surprised how many couples have never done this since their honeymoons.