The biggest reason for declining marriages is that people can actually make choices now and people don't want to be shackled to someone they hate their whole lives.

People had radically different attitudes about marriage in olden times, when marriage equaled survival in many ways, and survival of your family, your connections, your career and your status, which was held through generations. It wasn't thought of normally in terms of love and that's a very modern view of marriage.

Romance and marriage for love certainly existed, but that was usually considered fantasy or stageplay.