Call me a cynic, but I've basically observed three types of married couples.
1. They truly love (and more importantly, don't annoy the hell out of) each other and they stay together happily.
2. They grow apart and want to end it, but respect their promise and stick it out unhappily.
3. They grow apart and decide to end things.
#1 is reserved for the lucky few. #2 used to be more prevalent, especially in the Catholic tradition, but has given way to #3 in modern times.
People also get married for the wrong damn reasons. I'm 23, so a lot of my friends are getting engaged. "Oh my God, we've only been dating for 3 months but we're TOTALLY into the same movies and we both loooooove The Arcade Fire!"