They fail for many different reasons...
Some people get married for the wrong reasons... like for having a baby.
Some people let their addictions take them over and that is the only important thing in their life. (DING-DING-DING... this is why my 'rents divorced)
Some people cheat.
Some people lack communication.
Some people are lazy and don't wanna help out financially.
Some people beat the living fuck outta the one they supposedly love.
Marriage is a lot of work... some people don't like to work or have better things to do.
I mean, I could go on and on really...
When I see a couple 80 some years old who have been together for 50 plus years, I find that so sweet, but really... I think we'll be seeing less and less of that as time goes on. I have maybe two friends that have parents that are still happily married... the rest are like my parents... happily divorced.