Sorry to be a downer.
I was having a stalk look on Facebook. A girl I met when I worked abroad three months, lovely girl had been with her boyfriend three years. She had even moved to Australia to live with him, he was then flying over to France to see her. Has now put on Facebook that she's single.
One of my exs left me for another girl. On paper they sounded perfect together - same age, same friends, same career and even from the same city and yet they broke up 8 months later (I am a little smug about that though.)
A couple I know had a baby together, been together a year before that too. Got married when he was 2 and a month later they'd broken up.
Obviously not all relationships fail, and some do and after some time you get back together but it just seems like a large percentage of couples really don't make it.