I was with my wife (gf at the time obviously) for about 5-6 years before we got married... while things were the same in terms of the exclusive nature of the relationship and what not, marriage definitely brings a lot of changes. 1st, you're obviously living with one another. I doubt you were living with him at 15... But more importantly, I think it changes people psychologically. Suddenly you feel (which is obviously absurd) that the other person aint goin anywhere.... when you're dating, one can just say adios and that's that. Clearly our society makes it pretty easy to get divorced.... (otherwise the rate wouldn't/couldn't be as high) but it's still a pain in the ass... so we end up taking the other for granted for that and many other reasons... next thing you know, the Big D hits.
that's why, if I could afford it and there weren't kids involved, I think living apart (assuming you're still relatively close) would be ideal for certain types of couples... you'd be married but sort of act like you're still dating. you'd rarely see the other at their worst... and i think both sides would put more effort in to making it work. but obviously that's absurd... so you end up falling into 1 of 3 categories.
the 1st is the most rare of course... the almost ridiculously happy couple. most times, they're comprised of easy going types that rarely push the others buttons.... I actually know a few like this and... ugh.... they're so boring!! lol
2nd is clearly the most popular... constant ups and downs... yet they stay together.... good outweighs the bad...even if it's only by a little.
3rd... people that, for legit (or not so legit) reasons decide to bail.... maybe the guy likes to wear white tank tops.... or maybe homez thinks he can do a lot better