Many wedding traditions have disturbing origins. Brides first donned veils to prevent the groom from changing his mind about the marriage if he didn't like her face, and the garter toss harkens back to the days when guests helped disrobe brides shortly after the vows were exchanged. Modern couples may not give much thought to where they stand at the altar, but the traditional placement—the bride standing on the left side and the groom standing on the right—has a dark origin story of its own.
The reason couples about to be married stand where they do is a relic of "marriage by capture," according to Reader's Digest. In early European history, a groom would sometimes literally "take a wife" against her will, the will of her family, or both. A man would kidnap a woman from her home with his groomsmen there to fight off anyone who tried to stop them. At the actual wedding, the best man—a.k.a. the man with the best sword skills—would stand by the groom ready to jump to his defense in case any uninvited visitors arrived.
The groom had to be prepared to battle angry soon-to-be in-laws as well. Because most people are right-handed, the groom starting standing at the right side of the altar so that his right arm would be free to draw his sword at a moment's notice.
Marriage by kidnapping has thankfully declined in popularity in recent centuries, but the groom's strategic position on the right side of the room remains. Judaism is one religion that switches up this placement, and the reason comes from a different tradition: A line in the Bible reads “At thy right hand does the queen stand,” and in Jewish weddings, the bride and groom are treated as royalty.
[h/t Reader's Digest]