Question: Why is the president's wife called the First Lady?
Answer: James Buchanan assumed the American presidency in 1857 and became the first (and only) bachelor president. Since Buchanan did not have a wife, he turned to his charming niece Harriet Lane to take over the social functions at the White House.
During her uncle's presidential tenure, Ms. Lane became Washington's most popular hostess and she used her special position to promote certain social causes. Consequently, people began to refer to her as the "First Lady" since she was the president's niece---not his wife---and thus had no formal title.
Somehow the title stuck.