Because he wishes what is best for you and your (future) marriage and is responsible for seeing to your preparation for marriage in the church. Actually, Catholics view the seven sacraments as a way of directing encountering Jesus. Only two of the seven are sacraments of service, directed towards salvation of others: Matrimony and Holy orders.
Most people understand that it takes years of preparation to become a priest. There is a lot of overlap in what the Catholic Church expects of a priest and matrimonial couple. Most people do not think it takes any preparation to be married. The reality is, it takes a lot to be married, even more to become a sacrament. The weekend helps to prepare you for that.