It is a legal contract that is supposed to last forever because being a parent and a family member is a role that lasts forever with lifelong responsibilities. Hence the need to protect the legal family union.
Until recently, marriage was not associated with love; rather with the legal protection of the children and family assets. In fact, it was rare for those who in love to get married.
In any event, this started changing around the start of the 17th century and the concept of marriage and love were combined; putting a huge amount of pressure on the legal contract called the marriage.