I still have all the letters my high school sweetheart wrote me during an eight-month separation at the height of our five-year relationship. Eloquent 10-page musings on the daily grind of his KP duty in the local pen. That, and the occasional rock-album art-pencil sketch. He wanted to be an artist, or a rock star. Dealing hash oil was just his ticket. Too bad one of his regulars was a narc.

I'll never forget him. We talked occasionally over the years until his years of harder drug use and even harder living eventually took his life. At his funeral, several people told me how often he referred to me as the love of his life. Sad.

After him, there was Tim, the film guy from Van who hooked me on sushi, and Doug, who hooked me on some less healthy habits and put all my closest friendships to the test. Then came the only guy I've ever lived with, Mick — who brought me back down to Earth with a good bout of domestic bliss. Richard turned me on to world music, Ethiopian food, and the Internet way before it exploded, and John turned me on to what a complete fool I can be when I fall for someone less interested in returning the favour. Paul got me to like myself again.

The guy you first orgasmed with; the one you had really raunchy sex with; the guy who ripped your heart out — every relationship leaves its mark, something to take into the next one en route to what we have been led to believe will eventually be the one. Some lessons are easier (if he tells you on your first date that he's still working things out with an old girlfriend, run quick), others take time, and there are usually a few repeat offences (it took me years to learn to keep away from the mute ones and find someone who could actually express a feeling or two).

Exes aren't just practice runs, however. They are a connection to your past, reminders of where you've been, how far you've come. I still have a sweatshirt John gave me. Every time I wear the damn thing I am reminded of how I let the immature twerp break my heart — same principle as rubbing Fido's nose in his own poop.

I try to make a habit of eventually becoming friends with my exes. I consider it the minimum return on my investment. It can be tricky, though. There are no set rules on how much time must pass before this is possible. A loose mathematical equation — something like, the ugliness of the breakup divided by the combined level of maturity — equals how long you have to wait before you attempt this.

Because you know what happens when you try to pretend you're over it too soon. It starts on the phone, then you agree to get together. Once face to face, chemistry takes over, you fall for some crap about things being different. It's easy to get sucked in by familiarity and a selective memory. Then your sex drive butts in on the conversation and the next thing you know it's morning and you're making coffee for two and suffering a major emotional hangover from falling off the wagon.

And, of course, it's never as good as you imagined it would be when you sleep with an ex. Not just because you're drunk, but because reality has a hard time living up to fantasy, and the distance between you makes it impossible to slip back to that space in which you were once so comfortably nestled.

Then there's seeing your ex move on. It’s fun when you run into him with his new girlfriend, and she looks exactly like you. It’s not as fun when you see him gleefully happy with someone new. Especially unfair is that she gets to enjoy the improved model while you're stuck with the renovation bills. It's hard not to feel resentful.

Still, a relationship history is crucial for learning about what you want (or at least what you definitely don't want) from a partner, and the nifty thing is that, in the process, you learn about yourself. Consider it a lifelong research project. Call it the ex-files.