I am friends with a couple who have been married for 40 years. Some time ago I asked them: “What’s your secret?” The husband’s prompt response was: “Inertia!”

The word “inertia” carries negative connotations – passivity, inaction and a general lack of energy or motivation to do anything very much. However, the Oxford Reference Dictionary defines it as: “The property by which matter continues in its existing state of rest or uniform motion in a straight line unless the state is changed by an external force.”

I think this describes very well my friends’ long and seemingly happy marriage: their relationship continues, relatively unchanged, because no external force has impinged upon the life they have built together. This might be down to the vagaries of Fate or it might be that they are simply quite content to “rub along”.

I actually think it’s rather lovely that inertia has proved to be the recipe for this successful relationship, for it shows that marriage doesn’t have to be about torrid passion, adventure or emotional roller coasting. In today’s high-octane culture, newly-wed couples could learn a lot from this way of looking at marriage. A friend of mine’s father once described marriage as “a damn good friendship” and this, too, is as good a recipe as any for keeping a couple together. Friendship might not sound sexy, but it tends to endure a lot longer than lust.

In a society where many couples live together before getting married, walking down the aisle carries more expectation than ever before. Consciously or otherwise, people imagine marriage will change their relationship for the better – and when it doesn’t, they become disillusioned. Hence statistics show that couples who cohabit before marrying have a greater chance of divorcing than those who don’t.

My friends have proved that inertia works: the marriage ain’t broken, so why try to fix it? Their relationship might not be all singing and all dancing, but it is comfortable, secure and mutually supportive and that, in the long run, is what is important.

• Next week: When inertia gets in the way of starting a new life