26 years for me and my wife this year. We fell in love, started a business, bought a house, then got married. Worked together for 25 years, (most of them 6 days a week), and never had kids. We got lucky.
I think after 3 tries, I'd give up too. If you get married too soon, or if one in the couple starts foolin around or turns psycho, I can see getting divorced. But aside from that, I think people just get selfish and forget what they loved about the mate in the first place.
I'd have a hard time trying a second time. Lots of the people I've known who have been divorced two or three times never realized that they were a pain in the butt to be around all the time.
Your question is a good one... maybe people fall in love and get married before they've had the time to learn if the person is a good spouse.
In general, lots of reasons: love someone and want to be together a LOT, want children in wedlock, don't like being alone, financial or emotional support...sometimes just as an escape (my mother "ran away from home" that way, at FIFTEEN!!!).
My first marriage was kind of a convenience thing (although emotions were involved too) - we were both military and that was a way we could be (for the most part) stationed together. The things we had in common as a military couple didn't last as a civilian one. But my son came from that union, so I have minimal regrets.
In a sense, my 2nd marriage was a bit of a convenience as well. I loved DH (still do...lol), but really didn't want to get married. I'd been single/independent for over ten years, had my own home that I'd scrimped and saved for, a son in school that I didn't want to uproot, etc. I was very resistant to changing any of that. But driving back and forth (about an hour drive) several times a week got old (for both of us) after a while. And the schools in his area were excellent. So I took a leap of faith in remarrying, and other than the rough first year of transition, haven't regretted it.
I know we'd all like to believe (because it feels so nice) that most couples get married for love, but there's really a LOT more to it than that - even throughout history.