Rice has been a part of wedding ceremonies for many centuries because throughout the ages it was a symbol for fruitfulness, a wish for the couple to have great prosperity and many children.
But the rice was used in different ways by different societies. In some societies, the rice was worn or carried by the bride. In others, the bride and groom ate the rice together as the actual ceremony of getting married. In still others, the rice was sprinkled over the couple.
Then, too, the rice may have also been used as an offering to evil spirits, to feed them and satisfy them so that they would not bring any harm to the marriage.
So when you throw rice at a bride and groom today, you are actually practicing a very old custom to wish the newlyweds good things in the life they are beginning together.
In some rural English communities today, people throw wheat over a bridal couple to wish them the same good luck!