Redemption will come when all the souls that need to come into this world have descended from the upper world. This is one reason we rejoice at weddings, because they lead to the bringing of children into the world, and thus bring the redemption closer.

The joy of a wedding indicates the approach of the redemption, and this is the allusion in the Talmud: “The son of David will not come until all the souls of the body [in heaven] have been finished [have descended to this world]” (Yevamot 62a). This is the joy of a wedding; it is for the souls that will come into this world as a result; this is the union, and through it we come closer to the redemption. For this reason we say in the seven wedding blessings [the blessing of]: “Celebrate greatly…,” [alluding to] the building of the Temple, because from this joy, the Temple will be built. (Rabbi Levi Yitzĥak of Berditchev, Kedushat Levi, Likutim)

Further reading: For more on the wedding ceremony, see A Concise Guide to Halakha, p. 47.