The blessing made at the wedding is a blessing over joy. The Mitzva G-d in the Torah actually commands the chatan to give joy to his new wife for the first year of their marriage. Rather, he should be free to be at home for a year and bring joy to the wife whom he married. According to Torah law this grants him an exemption from normal army service for that year. The obligation to bring joy to one s wife is for a lifetime.
Sadness is connected with the world of destruction .
Joy has a powerful influence on the chatan and kalla at the opening of their marriage when they are building their home. The intense joy of their wedding brings them into the world of building, to build a house of holiness and purity. This is called building one of the ruins of Jerusalem. The unity of a man and a woman involves the Divine bringing into being of a new existence in the world. This is because their unity is a Divine one.