Experts now have a new term for relationships which are not obviously dysfunctional from the outside and rarely seem in danger, but are devoid of growth, passion or a fundamental connection.
They are relationships on 'autopilot', a state that they say can prise couples apart as easily as blazing rows and even infidelity.
"In a long term relationship we can't intensely focus on each other all of the time," says relationship specialist Rinatta Paries. "Life often pulls our attentions elsewhere and this is good and healthy for a relationship. It gives both partners the opportunity to grow and bring back something different to the relationship.
"The problem is that many couples end up spending most if not all of their relationship in this autopilot mode and this eventually kills the joy in their relationship, and perhaps even the relationship itself."
Psychologist John Grohol is even more pessimistic. According to him, "a relationship can survive most things if both people involved in it are committed to the other person and act with respect toward the other...
"What a relationship has real difficulty surviving is when two people have gone into autopilot mode and become indifferent toward one another."
Are you being caught unaware?
The problem with autopilot mode is that, unlike infidelity and anger, it's easy to slip into without realising. Mix the mundane routines of life with an unrealistic view of the effort needed to make relationships work and you have a recipe for indifference, and eventual disaster.
Experts say that, after the romance and fireworks of the earliest years, too many of us forget the golden rule of any long-term partnership. To survive, it needs work. To prosper, it needs to grow.
"There's a mentality (among some couples) that once they've 'got' each other all the work of the relationship is done and they can relax and focus on other pursuits in their lives," says Rinatta Paries. "Nothing can be further from the truth and yet this idea persists."
Of course, no long-term relationship can be one endless round of deep conversations and earth-moving sex. Life, routine and familiarity get in the way. But relationships need to offer more to those involved than what Paries calls, "the comfort of routine and the false sense of relationship security."
Experts agree that we all go into autopilot mode once in a while, particularly when factors outside the relationship work, studies, family problems exert a bigger pull on our attention than normal.
But if we stay in it too long, or it becomes the normal state of the relationship, we risk missing out on the things we got into a relationship for in the first place.
"Since we get into relationships to get 'relationship goodies', such as love, joy, a play mate, satisfaction, fun, excitement, adoration, appreciation, etc, the absence of these is a very dangerous state indeed," says Paries.
Couples on autopilot are often subconsciously (or otherwise) weighing up other options. Suddenly, single life looks pretty rosy again, because the benefits of being part of a couple are largely missing. Third parties whether an affair is real or fantasised seem to offer what the relationship has lost.
So how do you avoid falling into the autopilot trap? Psychologist Sue Wright says you have to look out for the early signs of a drift into indifference.
"Those may be that you are not spending quality time together, and are starting to not communicate and discuss things - you are no longer talking about the things that matter to you, and you are starting to become isolated within the relationship."
At this stage, says Wright, "none of this means that things have to end." But, she adds, now is the time when "effort has to be made." If you don't fix things now, later may be too late.
If you do see the signs, the prescription is pretty simple. "Simple things are important," says Wright, "like making sure you go out together from time to time, that you create an interest that you both take part in, or that you sit down with each other for dinner and talk, rather than watching television."
Rinatta Paries says you need to become 'attuned' to your partner. "Assume that you never completely 'have' your partner and keep working on keeping him or her by listening and attending to his or her needs and wants." Your partner should do the same, of course.
Both experts agree that finding time for each other in busy schedules is key.
"Go ahead and get involved with your life, work, hobbies, children, etc, but remember to always come back to your partner willingly and eagerly, lest you lose him or her," says Paries. "Let him or her be your touchstone, the place you return to for comfort, joy and pleasure."
Which is pretty much the opposite of autopilot, of course.