Most people don't take the time to do some serious introspective thinking about what they want in a mate for life before getting involved with someone. Marriage is a choice. If we are not happy with our spouse it means we aren't happy with the "choice" we made.
The majority of people aren't willing to invest the amount of time it takes to truly get to know someone. They become engaged while they are still in the "infatuation phase" of their relationship. From there on it's like a snow ball rolling down hill towards the wedding day. Even if one or both are secretly having some doubts they attribute it to having "cold feet" and press on. After all the pomp and circumstance the dust settles and they realize they made a mistake.
They now see they didn't discuss many things which were important to each of them.