There are many lists of tips and tricks for dating, and many opinions about what to look for in a spouse. But I’ve decided to boil everything down to one simple idea, based on the hundreds of combined years of my patients’ lives that have been shared with me in my psychology practice, and my own experience of married life.

To understand the most important quality to look for in a spouse, you have to grapple with this simple fact: Being married will be unimaginably different than being single.

That’s because the single life is defined by independence. Although you may feel like you spend all your time with other people, if you are single your life is yours alone. You answer to no one but yourself. This is radically different than the life of a married person.

If you commit to your marriage vows, then getting married will mean giving up your independence and sharing your life with another person all the time. And that’s not all—it means living with this one person for the rest of your life.

One person to live with, make plans with, face life’s difficulties with, raise children with, move different places with, earn and lose money with, eat good and bad food with, etc., etc., until death do you part. That’s a long time.

It’s difficult to conceptualize how long the rest of your life might be. Try this: think back to when you were 3 or 4—your earliest memory. Was it twenty years ago? Thirty? Maybe more. Now think about how long that feels like, and project that length of time into the future. Now double it. That’s how long you will be living and breathing and sleeping (and not-sleeping) next to your future spouse.

So while you have your independence, use your time to seriously consider all the qualities—attraction, virtue, holiness, family background, life goals, etc.—that are important when choosing a spouse.

But once you get married, you’ll realize that there is just one quality that matters most for the long haul.

Only friendship makes the crazy demands of marriage humanly possible.

Only friendship makes it worthwhile to give up the radical independence of being single.

Only if the person you marry is your best friend will you be truly happy with them for the rest of your life. This is really the most important—if not the only—thing you need to think about when you are looking for “The One.”

What about attraction; does it matter when evaluating a potential spouse? Sure it does—to a point. But you shouldn’t have to think about it. If you’re wondering, “Am I attracted enough to him/her?” then you are not mature enough to be choosing a spouse. If you consider deeply the reality of marriage as I’ve outlined above, you can be assured that the physical beauty and the butterflies that attracted you when you were dating are not what will keep you sane for the rest of your lives together.

There is a deeper aspect of love inherent in the attraction question. As you get to know and love a person more, you find the person more beautiful. As you give yourself fully to your spouse, you open yourself to receive more of him/her than you’ve ever received of anyone in your life. You have no idea how beautiful you will come to see your spouse if you truly love in this way.

In many ways love between spouses begins on the wedding day. That’s why so many couples look back to the time they were dating and laugh at how naïve they were. The wedding day is the first day that your body and mind express together the totality of self-gift to one another. That is the first day of a lifelong adventure, which is growth in love—and holiness.

Yes, holiness matters. But if you’re “looking for your St. Joseph,” or you’re looking for someone “as holy and pure as Mary,” then you’re not ready for marriage—especially the messiness of marriage.

Marriage is a vocation; a vocation is a calling from God to live your life in such a way that you “seek to lose yourself that you may find yourself.” In other words, it is a path to holiness. That means that you don’t find a holy person to marry; you become holy through marriage. That means that you are far from holy before you get married. And your spouse will be far from holy when you get married, too.

God willing, after a long and fruitful married life together, you both will have grown in holiness together.

So what about the personality tests, the checklists of interests, the cliché Match.com questions and everything else that sells magazines? All of those can be summed up in one quality—friendship.

If you find a person of the opposite sex with whom you can be yourself; if you can imagine taking a cross-country road trip with them without jumping out of the car; if you can wholeheartedly will their good; then you just might have found “the One.”

Of course you can’t know that immediately. But as you plan dates with each other and other people, you can ask yourself: Will this feeling of comfort and willing of the good last over time? Can I make a reasonably good guess that I will feel this way in a year or two, now that I’ve developed a deeper relationship with this person?

You’ll know it when you find it.