It isn’t necessarily supposed to last forever. Obviously most people want it to when they get into it, and the majority of first marriages actually don’t end in divorce so apparently most of them are doing it right, but there’s nothing in the definition demanding that it last forever. I mean, people die, so obviously marriage is lasting forever. People change, people make mistakes, people betray each other. There are plenty of legitimate reasons to end failing marriages. Grow up, life isn’t a Disney movie.