Legend says that the holiday became Valentine's Day after a priest named Valentine. Valentine was a priest in Rome at the time when Christianity was a new religion. The Emperor at that time was Claudius II. He ordered the Roman soldiers NOT to marry or become engaged. Claudius believed that as married men, his soldiers would want to stay home with their families rather than fight his wars. Valentine disobeyed the Emperor's order and secretly married the young couples. He was eventually arrested, imprisoned, and executed.
After his death, Valentine was named a saint.