Why does desire become inhibited in so many long-term relationships? The conventional explanation is familiarity. But this would oddly single out sex from every other enthusiasm we have – Indian food, jazz, Rembrandt’s self-portraits – none of which lose their appeal simply because we know them well. I find familiarity an unconvincing explanation on its own.

At the beginning of a relationship, we do not yet know or love the other person, and sex is often easy. As love and knowledge deepen, sex can become more difficult. The problem seems less about familiarity as such, and more about a tension between eroticism on the one hand, and love and knowledge on the other.

And yet, when desire falters, the solution we are encouraged to pursue is more intimacy: more openness, more sharing, more closeness. Little wonder that when this is taken as the cure, the problem often intensifies.

What grows is not familiarity, but anxiety — and anxiety is the enemy of desire. Anxiety arises as we get closer to another person and begin to encounter something unsettling: that however well we know our partner, it is impossible to fully know what they want, or what they want from us.