We did the classic thing of growing apart.
The first few years of our married life were great fun. I had a job with a bank. Kate worked for a food magazine. Any occasional misunderstandings we had were soon brushed under the carpet and forgotten. We had time and money and few responsibilities. Life was good.
Then came children. Kate was a natural mum, so good that it was quickly obvious that she should make all the decisions about routines, clothes, food. I loved my two new babies and loved being involved as a dad. But with Kate in control at home, it was easy to take a back seat and focus on earning what we needed.
The drift was ever so subtle.
As I gradually stopped taking the initiative for what needed doing at home, the steady stream of requests - 'can you do this', 'can you do that' - slowly started to wear us down. Kate felt increasingly frustrated that I wasn't noticing what needed doing. I felt increasingly got at.
Our conversations became fewer and more stilted. I hid in my work because it was challenging and rewarding. I could kid myself that being a provider was for the family. Meanwhile at home, Kate felt neglected and alone.
With Kate's eyes on the children and mine on work, we failed to notice that nobody was looking after our relationship. We had done the classic thing of growing apart.
Eventually things came to a head. In desperation, Kate wrote me a letter. I found it lying on my bed when I got home from work. It read like a job spec of what it was like to be Harry's wife: terms and conditions, perks, travel, etc.
The letter ended with the awful sentence: 'But what I really want is our friendship. Will I ever get it? Who knows. WHO CARES.'
Those last two words in capitals blew me away. Something shifted in my head. Suddenly I realised how I'd neglected my wife. I went straight to see Kate and dropped to my knees. 'I am so sorry,' I said. 'You have no reason to believe that I will change, but I will.'
The mental shift I made in that moment was tiny. But its effects were seismic. Instead of wanting the marriage to work for the kids or for me, now I wanted it to work for Kate.
I now started to notice Kate. Soon afterwards, I overheard her tell a friend that I hardly ever complimented her. It had never occurred to me that this could be a problem as I had never especially needed compliments myself. But I paid attention and put a post-it note saying 'compliments' on my computer screen. Kate laughed when she saw it. 'A one week wonder,' she thought. But two years later when the note lost its stickiness and fell off, the pattern had been established. The other day I heard Kate tell a friend that I compliment her all the time and that she loves it. Compliments have now become so automatic that I barely have to think about it. I do it because I notice Kate.
Kate also once told me that what made her feel loved was when I hung out with her and chatted with her. Again this isn't natural to me. I'm comfortable in my own skin and quite happy on my own. Recently we had a family bereavement that came out of the blue. I knew Kate needed my support so I deliberately spend time hanging out with her. I could just as easily have grieved on my own. It took a decision to get up, leave my desk in my office at home, and just be with her. Yet it's made the grieving process a little less awful knowing that we are close as a couple.
I read that letter twenty one years ago. We have since had four more children and have just celebrated our thirtieth wedding anniversary. Ironically, I've spent most of those years teaching thousands of couples how to have a great relationship and researching what works.
Yet until now I've been careful not to assume that our experience is any way typical. But it is. There are millions of Harrys and Kates out there who needn't get into the mess we had.
Realising how parenthood changes us as a couple is the key to a successful marriage. Becoming parents changes the game and sets the tone of a marriage for years to come, for better and for worse.
Forget Mars and Venus. The main difference between men and women is that women have babies. Nine months of pregnancy shifts a woman's thoughts and orients them to her baby automatically. She can't help it. It's human nature.
So it's all too easy for a new dad, even one besotted by his children as I was, to take a back seat. Mum makes the decisions and dad does what he is asked. But it's just a question of time before this begins to grate on both parents and the drift begins.
Somebody has to look after the relationship. It's no good saying it takes two to tango. If you think both of you are equally responsible, then nobody will be. But if the husband can see that his first task is to love his wife - whose eyes have now shifted down to their baby - then she can love him right back. In that order. It takes one to lead.
Our experience turns out to be hugely typical. My own Marriage Foundation research shows that two out of three parents who split up were at least somewhat happy and not quarrelling much only a year before their split. That's a lot of Harrys and Kates whose relationships ought to be recoverable.
What mums want, according to the survey we ran for our book, is a friend, somebody interested in them and the children, and somebody who is kind. That's it. If we dads can understand that, it could revolutionise family life. Most breakdown is far more avoidable than anyone currently imagines.
Husband, love your wife. And she will love you right back. That's what mums want. That's what dads need to know.