They just don't know any better. Research has shown that couples who live together before marriage divorce more than couples who don't. Children of divorce and other tentative family arrangements have higher rates of truancy, drug abuse, sexual promiscuity, emotional problems and academic underachievement. Understandably, these children are reluctant to follow their parents' path. So marriage may be one of society's insoluble problems.
Marriage is hard work, and our expectations for happiness are nearly impossible.
It is axiomatic that people who just want to have fun are doomed to disappointment and failure. We can probably agree that marriage is good and necessary to our national health and that our goals should be set accordingly. But we might also acknowledge that forever is a long, long time. To say that marriage may need some adjusting isn't to deny its importance. But problematically, the knots we tie today are bound with the same frayed strands from an era when newlyweds were lucky to live another 20 years.