Q. Why do a bride and groom fast on their wedding day?
A. There are some occasions like Rosh Chodesh when this does not happen, but the general principle, enunciated in the Magen Avraham, a commentary on the Shulchan Aruch, is that the wedding-day fast is for the purpose of forgiveness of sin and to ensure the couple come under the chuppah in a serious frame of mind.
The Rokeach adds that the wedding ceremony is a religious act, and just as one should not eat before religious duties such as hearing the Shofar on Rosh HaShanah, the couple fast until after the wedding ceremony.