Stanley Hauerwas, an ethicist at Duke, says that we always marry the wrong person. The sooner young couples can understand that, the better off they'll be. I hear young couples say, "You mean you don't want us to be soul mates?" But nobody marries his or her soul mate. You become soul mates by living life together through those years.
So often cohabiters are looking, in the first year, for what comes only after years—decades!—of life together. You are setting yourself up for dramatic disappointment if you think life works that way.