The tradition of carrying the bride over the threshold is not a new tradition; it dates back centuries and has a few different origins.
The first idea for carrying the bride over the threshold stems from the same ancient belief as the idea of an aisle runner and throwing flower petals on the aisle. This belief is that the newlywed couple is very susceptible to evil spirits. By carrying the bride over the threshold, the groom is putting a protective space between her and the floor; thus, protecting her.
Generations ago, it was considered lady like for the new bride to be, or at the very least appear to be, unwilling to "give herself" to her new husband. Whether she was or not was not the issue, it was all about appearances. At the threshold to the bedchamber, the husband would have to carry her over to encourage her to go in.
Another tradition dictates that the new wife must enter her home for the first time by the main door and to avoid bad luck, she must not trip or fall. To avoid this, the groom would carry her into the house.
During the ancient times when people were married by capture, the bride obviously would not go willingly into her husband’s home. So she was either dragged or carried in. Over time it evolved and became the tradition that is practiced today.
Even earlier times believed that family demons followed the bride to her new home. To keep them from entering the home, the groom would carry her across the threshold the first time she entered the home. After that demons could not come in and the bride was free to come and go as she pleased.