Today, weddings must be legally witnessed and we now assume this is why the groom must choose a best man. Here is a much more interesting reason.
The beginning of the best man can be found with the Germanic Goths, for whom it was customary to marry within their community. When women were in short supply, bachelors would seek out and capture brides from other communities. Unsurprisingly, this was often met with hostility and so the bachelor would have to choose the �best man' he knew to help him fight off any opposition.
The tradition was still operating in around 200AD as after the bride had been captured, the groom was still at risk as the bride's family may try to forcibly retrieve her. Therefore the best man remained at the groom's side throughout the wedding - well armed of course. The best man continued this protection by standing guard outside the newlyweds' home.
While these theories are founded in German folklore, discoveries at many ancient churches have uncovered a supply of clubs and spears, supposedly to ward off an attempt to reclaim the bride during her wedding!
Also stemming from the capturing of the bride is another common wedding tradition used today. Traditionally the bride stands to her groom's left, this is thought to be because the groom would need his right hand free to draw his sword and protect himself and his lady.
While the origins of these traditions may be slightly misty, they do add another dimension to the marriage we take for granted as common place at modern marriages.