Today, the supposed key ingredient to a happy marriage is being “best friends.” No doubt, there has to be elements of friendship in a romantic relationship. A couple needs to be able to trust and support each other, laugh together and confide in one another. But friendship is only a fraction of the expression of an intimate relationship, in fact it’s the lesser expression.

Here’s why.

Friendships are characterized by closeness, support, and trust. So are romantic relationships. But other facets of friendship, like total transparency and access can actually undermine what really keeps two people together: Lust.

In a marriage, lust is the top priority. two people choose each other, not just to share of themselves and feel supported in a friendship, but to lust for and be lusted after. People want this intensity, even though it has the potential for more extreme heartache and rejection. The powers of touch and desire instill a sense of longing that cannot be rivaled, a feeling of being alive and living life at the mountain’s summit.

Sure, you want to be able to tell your spouse everything and feel completely known and effortlessly understood. But these are the qualifications for friends, not lovers. Only with your lover will you be able to peel back endless layers and still be left wanting more. When done right, lovers, even years into their shared life, still feel a little tension and mystery between them. The calm waters of friendship are no match for the fiery blaze that keeps two people reaching for each other year after year.

The foundation of a marriage is built, not on shared experiences, but on shared hunger for one another. At its core, marriage does not stand on shared interests or commonalities but on the magnetic, mystifying pull between two people. You may get deeply attached in friendship, but only desire can bond two into one.