Why is a wedding ring worn on the
third finger of the left hand?

As far as we know, the ancient Egyptians were the first to place
a ring on the third finger of the left hand to signify the
marriage union. It was placed on that finger because Egyptians
believed That the "vein of love" ran from this finger to the
heart. They used a ring because they believed that the circle was
the symbol for eternity. It represented perfection because it had
no beginning and no end.

Rings found in ancient Egyptian tombs were made of pure gold. The
name or title of the owner was engraved on the ring in
hieroglyphs. The poorer citizens of Egypt wore rings of silver,
bronze, amber, ivory, or simply glazed pottery. Because gold was
precious to the early Romans, a gold ring symbolized everlasting
love and commitment.

King Edward VI of England decreed that the third finger on the
left hand was to be the ring finger. In the 1549 Book of Common
Prayer, the left hand was designated as the marriage hand. From
the earliest times in our history, people have always given
advice to newly married couples such as "comfort each other,"
"respect one another," and "listen to each other." One of my
personal favorites is "Never yell at each other unless the house
is on fire!"

~source used: "What Makes Flamingos Pink?"
by Bill McLain