Seems like it's a common story here, but for me it was a one-night hook up from the club that turned into something more.
Both of us had just broken up from other relationships, and wanted to go out in towns that were not our home towns (we didn't live in the same towns) so that we wouldn't bump into our exes by accident. Those other towns happened to be the same town, so we met and hooked up at the club.
The night was disastrous from a sexual point of view - vomit, unremovable contact lenses, shit on the sheets - but we got along well enough that I offered him my phone number the morning after. Then, to my total surprise, he actually called me. We set up a date to visit the next weekend, which we did. I was still at university at the time, so for a year he drove over to see me every weekend. After that year I moved in with him, and that was 14 years ago.
Because we couldn't be together all the time for that first year, the relationship grew fairly slowly. But I do remember that I went through a phase of, whenever he was due to arrive, somehow managing to injure myself through sheer nervousness. Stubbed toe, cut finger, twisted ankle, something.
I also remember one time when he was late, and I was pacing the room, couldn't settle until he was here. It really pissed off a friend of mine. Once he finally arrived, I remember sighing and relaxing. It was like an addict's reaction - jumpy while waiting for your fix, blissed out once you've taken it. I think that was when I realized how deep I was into this.
We do have an open relationship, but it took a while to develop to that point. We started having threesomes after a couple of years, continued in that vein for a while. Eventually we came to understand that we each had interests that the other didn't share, and we agreed that we could explore those interests outside the relationship as long as we were safe and there was no emotional component to it - it was just sex, not another boyfriend. Granted that hasn't always been able to be maintained, and it did cause some problems before we found the right balance. Where we are now, we have an open relationship in theory, but in practise we mostly can't be bothered. It's too much effort!
After all the crap we've been through in our relationship - both self-inflicted and from outside influence - we've faced every situation, and while we have been tested, nothing has yet succeeded in breaking us up. I don't think anything is ever going to manage it now.