Can I clarify what you mean by a 'sudden change'? While it depended on the country/city state and the class of the couple, marriage during the Early Modern period was more of a family arrangement than a private one. This was consistent across the centuries, although the importance of marriage becomes especially stressed after the Black Plague, when it was crucial for the population to reproduce. It's also difficult to stress a dichotomy between love and marriage, as arranged marriages between couples weren't necessarily devoid of love. A number of sermons from the time stress the importance of love between a married couple (see San Cherubino and San Bernardino’s public sermons). If, however, you take a modern view of love, this is actually far closer to the idea of courtly love than it is to the idea of marriage. Passionate romance was as much an intellectual pursuit as it was an expression of emotion, and women engaging in the game of courtly love were usually already married.

A good start on the idea of romantic love is Medieval Misogyny and the Invention of Western Romantic Love by R. Howard Bloch (although it is a controversial text in some ways!)

Also a comprehensive study on marriage in Europe: The Development of the Family and Marriage in Europe by Jack Goody.