Why is 13 considered an unlucky number?

There are at least two possibilities, according to the people who claim to know such things. One is that there were 13 people at the Last Supper, and one betrayed Jesus Christ. The other is that in Norse mythology, 12 Norse gods were enjoying a feast at Valhalla when a 13th — the cruel, mischievous god Loki — intruded and caused the death of Balder, the embodiment of joy and gladness. In some parts of the world, 13 means nothing. In Madagascar, 6 is the unlucky number; in Japan, it’s 3. And consider these fortuitous 13s: 13 original colonies; the Great Seal of the United States contains 13 stars, 13 bars, and an eagle that has 13 tail feathers and is holding 13 arrows and an olive branch with 13 leaves (“E pluribus unum” even has 13 letters); George Washington laid the White House cornerstone on a 13th, and the cornerstone of the Supreme Court Building also was laid on a 13th.