1. The Relax Factor - Whenever we are in pursuit of something or have a goal of some kind we put in our best efforts to beat out the competiton. We take a keen interest in learning as much as we can about the person, their likes and dislikes. We listen intently and look for queues to tell us how well we are doing and so forth. Unfortunately there are many people who view marriage as being the time when you can STOP putting in your best efforts! After all he or she is "committed" to you now. This makes little sense.
Just because you were hired for the job doesn't mean you don't have to perform well in order to keep it! No one is "stuck" with anyone. When we change our circumstances change.
2. Expectations - Many of us (expect) passion and romance to fade! It becomes a self-fulfillng prophesy. Some of us have been taught to believe that "real love" is more about companionship and has less to do with physical intimacy. It's not uncommon to wake up one day and realize you have become more like siblings or best friends. You have completely desexualized one another. We put our focus on the kids, careers, hobbies, friends, and family. One day someone has an affair or announces they want a divorce. All of a sudden sexual intimacy becomes important and we try to recapture what we allowed to die. In reality what separates (romantic love) from parental, sibling, friendship, and sibling love is our sexual desire for our mates/spouse. "It's easier to maintain a fire than it is to reignite a spark!"
3. Evolution - People change. What you may have wanted in a mate age 25 may not be what you want at age 35 or 45. There is no neutral in marriage! We're either "growing together" or "growing apart". It's not uncommon to hear someone say, He/she is not the same person I fell in love with. When two people no longer want the same things from the relationship it's pretty much over. Just because someone is "going along to get along" doesn't mean they are happy with the way things are. It's important to check the temp of the marriage periodically without putting the other person on the defensive. Always find something to "look forward to" as a couple.
4. Settled - A lot of people get married for the wrong reasons or they made a bad choice in selecting a spouse. Not everyone who gets married should stay married to the person they chose. Maybe marriage was goal by a certain age, accidental pregnancy happened, alll your friends had gotten married, or you found someone who was crazy about you and although you didn't feel the same about them it seemed like a safe choice. This sets up the old, "I love you but I'm not in love with you" speech when you decide to depart. Staying together for the wrong reasons is just as bad as getting married for the wrong reasons.