A chance encounter this week has left me in nostalgic mood. But sadly I’m not enjoying it.

Unexpectedly, I bumped into a former colleague I last saw about 25 years ago. It came a few days after my family and I were remembering my father on the 10th anniversary of his sudden and unexpected death, which, as it does every year, takes me back to my extremely happy childhood in our home in an East Midlands village.

But this more recent encounter has stirred up very different feelings, and I’ve found them quite disturbing.

That same evening, I found myself dragging storage boxes off the top of my wardrobe and flicking through old photographs until I found the one I vaguely remembered of my former colleague at a party at my house when I was 21.

However, instead of inspiring a rush of happy nostalgic warmth, sadness came over me, a marked contrast to the feelings inspired by memories of my father. So strange, because 25 years ago I was enjoying my job and life generally.

So why the sadness? I have put it down to the same feelings I experienced when I attended school reunions in my 20s. I thought I’d love seeing my old schoolfriends again and reminiscing about our school days, which for me were happy ones.

But I hated every second off them. I remember on both occasions deliberately getting drunk as quickly as I could to blot out the entire event, and vowed never to go to one again, even though my secondary school is in the village where my mother still lives.

I have deduced that it’s the time lapse effect. At the school reunions, I was seeing schoolfriends for the first time in more than a decade – and everyone had aged and changed.

Of course we all had, it’s only natural. I don’t know what I actually expected but it was a shock. They were no longer teenagers but adults with careers, marriages and children.

Instead of cheering me up, the reunions left me depressed because it was a reminder that time had passed, that our lives were 20 years on, and that many of my old schoolfriends appeared to be happier with their lives than I was with mine. So jealousy may have played a part in my down mood.

And more than a little bit annoyed? Yes, I was. Annoyed because when at school I was shy and academically average, but left school determined to do something with my life, to show them all that I was actually better than they thought I was.

However, I must have felt I hadn’t achieved that yet, still too young to understand that happiness comes from within, not from the way we perceive other people perceive us.

The time lapse effect kicked into action again last week. This time the gap was 25 years, rather than 10 or so, and so the effect was sharper, especially as my former colleague and I both ran through the story of our lives in a CV style during our brief conversation.

Later, I realised it had dragged me back to more youthful days, a fact made painfully obvious to me as I flipped through those old photographs.

Sometimes, I’m sick to death of my own past, sick of reiterating edited highlights according to the circumstances: meeting new people, meeting old friends, seeing distant relatives for the first time in years, job interviews, and so on.

And as you get older, it seems it’s your more distant past, the one you’ve been over millions of times already, that is the one people are more interested in. Middle-aged contentment is no competition.

By contrast, memories of my father, more intense around the anniversary of his death, are constant, and so the time lapse effect fails to materialise. Thank Goodness.

He remains the same, whether my memories are of him as a younger man during my childhood, a middle-aged man when he became a grandfather, or at 64, the age at which he died. No more surprises, please.