A lot of marriages don't work.

Many of these end in divorce. Lots of others just limp along for decades - convenient cohabitations that never really realize the potential of a well-functioning marital relationship.

In fact, researchers estimate that only about 20 percent of marriages - two out of 10 - really meet the expectations, fulfill the hopes, make real the dreams of the men and women involved.

That's the bad news. The good news is that we've spent some time and energy studying this 20 percent and we've got some pretty solid ideas about why they work.

A well-functioning, expectation-meeting, hope-fulfilling, dream-realizing marriage is nothing more nor less than a "romantic best friendship."

Romantic - that special blend of physical and emotional attraction and intimacy between two people that we experience so profoundly in courtship, and which we need to experience throughout the life of a marriage.

Best friendship - that slowly developing bond between two people grounded in shared interests, mutual experiences, common commitments and reciprocal caring.

Romance is often a function of our differences (opposites attract). But friendship is usually a function of our similarities.

Obviously there is a tension between difference-based romance and similarity-based best friendship. Research suggests, though, that it is the best friendship which is the key.

That may seem obvious. We all know that romance waxes and wanes over the life of a relationship. Sometimes life just gets in the way - jobs, children, illnesses, etc., all can distract us from our romantic focus. Sometimes our differences even become more sources of distraction rather than romantic attraction. Though romance is necessary for a healthy marriage, it is not sufficient. A romance-only marriage just won't last.

On the other hand, a true best friendship perseveres. Life's challenges focus and strengthen such a friendship. Friendship discovers and develops even more similarities and common bonds over time. Best friendship is the one absolutely necessary ingredient of a healthy marriage. We need romance; we have to have friendship.

If you are planning a marriage, ask yourself the question: Are we best friends? If not, the odds are against you.

If you are already married, ask the same question. And if you and your spouse are not best friends, then devote all the time and energy you can to building such a friendship. Do it now; it's never too late.

• The Rev. Ken Potts is a pastoral counselor and marriage and family therapist with Samaritan Interfaith Counseling Centers, Naperville and Downers Grove.