Shakespeare called it "the slings and arrows of outrageous fortune". In the 1970s we just called it the fickle finger of fate; the result was the same.
Just when you were ready to buy your first brand-new bike, that Yamazuki 1100 you'd been lusting after since puberty, the adventurous young lady who'd been adorning your pillion seat said: "I'm pregnant - we gotta getta car."
And that was that: Superbike Sam the breakfast runner became Suburban Sam, with 2.3 kids, 1.4 cars and 1.2 bathrooms. Biking days gone for good.
But that was then. Now the gyms are full of fiftysomethings who are fitter than I was at 20, children safely out of the house and with disposable income to burn.
Some of them go out and find that Yamazuki 1100 in a scrapyard and restore it to way better than new condition. They wear anoraks, bend your ear endlessly about the good old days and, sometimes, even ride their new old bikes. They're the backbone of the classic bike movement and we salute them.
But others, more into riding bikes than restoring them, spend a small fortune on the latest sports machinery and state-of-the-art kit, get out there on a Sunday morning and crash their brains out. Insurance companies refer to them unsympathetically as "born agains" and complain that they have the biggest accidents and the most expensive claims.
And the reason isn't very hard to find: they figure that riding a motorcycle is like riding a bicycle - you never forget how. Not so.
A 1970s litre-class musclebike weighed around a quarter of a ton with a full tank and delivered (factory figures notwithstanding) about 60 genuine kilowatts at the back wheel. Today's superbikes are good for more than 150kW and weigh considerably less than 200kg. Thanks to high-pressure fuel-injection and variable-length intake tracts, they have the throttle response of a striking cobra and they turn in like a terrier after a rat.
No matter how fast you were 30 years ago, a modern superbike is a whole new ball game. Everything happens much more quickly and the consequences of getting it wrong are much more severe - but try telling that to Superbike Sam, who could easily outrun all his mates back in the day.
At a recent track day I saw several riders I wouldn't hesitate to describe as born-agains, wearing brand-name kit and riding the latest and greatest in sports machinery - yet they were wobbling round trying to square off the corners like a 1970s Grand Prix racer.
A little ludicrous? Maybe - but the point is, they were in exactly the right place, where there were no kids, dogs, taxis or little old ladies swanning through stop streets without looking. They were making a quantum leap into the future of motorcycling under the safest possible circumstances, learning that, on a new-millennium motorcycle, finesse is a survival mechanism - and having a whole lotta fun in the process.
Welcome back, guys. Ride safe.