Friedrich Nietzsche wrote the line more than a hundred years ago and nobody has improved on it since: "It is not a lack of love, but a lack of friendship that makes unhappy marriages." The German philosopher, author of "Thus Spoke Zarathustra" and "Beyond Good and Evil", was not writing a relationship guide. Which is exactly why the sentence stings more than any guide.

Psychologist Lara Ferreiro explains it with a formulation anyone who has been in a long relationship will recognise: "There can be love, and still be distance, loneliness, misunderstanding or anger." Two people love each other, live under the same roof, and feel alone. That is not a paradox - it is the most common diagnosis.

Friendship within a relationship, she says, is not tenderness. It is trust, shared laughter, support and the feeling that "the other person is on your side". Notice how little of that has anything to do with romance. Not one of those four elements requires candles, a dinner or an anniversary.

Ferreiro names the three things that die first. The first is emotional closeness - the moment you stop speaking honestly because you are afraid of being laughed at or judged. The second is curiosity: after a few years you are convinced you know the person beside you completely, and people keep changing. The third is admiration - what once seemed most beautiful about someone becomes invisible, and criticism takes its place.

There is one piece of good news in all this. Arguments are not a sign that a relationship is failing. What decides things is how you behave while arguing and what you do afterwards. A relationship is not measured by the number of good days, but by how fast it repairs after a bad one.

Ferreiro's answer at the end is disappointingly simple and, precisely for that reason, credible: small everyday gestures. Asking how their day went. Saying thank you. Spending time together with no phone on the table. None of it costs money, none of it gets posted, and that is exactly why it is the easiest thing to forget.