Volatile state of marriage

Today I had to go and wait in a rather long queue in a municipal office to obtain a copy of my marriage certificate, or as Italians call it "stato di matrimonio", the state of marriage. Everything sounds better in Italian, doesn't it? In the printed form one has to fill when applying for the certificate one must state the purpose for which the certificate will be used. Most of people in the queue before me were applying for the certificate in order to get a divorce, as I couldn't avoid finding out from the office employee who was routinely checking the correctness of each filled form in a carrying voice.
I don't know what hit me. The thought that the same two people who now want to be free from any bonds with each other just a few years ago wanted nothing but to grow old together, to close an eye on imperfections of each other, to support and to carry on carrying on. Also the fact that three out of five marriages end up in divorce. The fact that there are so many people who after divorce have to reinvent themselves and their social life accordingly, to search for comfort and support elsewhere, or to cry themselves into sleep every night because they keep failing at finding it and start losing faith. The fact that it's easier to leave than to be left behind. I've seen it all among my friends.
I also felt thankful (to myself?) that I haven't given up on the man who drives me bonkers more often than not, who at first sight doesn't seem to be willing to apply himself to make me happy, but is unhappy if I search for things that make me happy if it doesn't go along his daily routine. I want to adopt a dog, he doesn't. I'm not a cat person, yet I've ended up with dozens of cats he adopted over last decade. I am blabbermouth, he's the biggest introvert I've ever met. We're a living proof that opposites attract.

Just like pain threshold is different for each person, I suppose incompatibility tolerance level is different for each of us. Someone wouldn't tolerate things I've compromised, someone else tolerates much more than me and doesn't even wince. There is no moral to this story, just the fact that I keep chewing on it - why people stay together no matter what? And why others don't?