How does the Valentine's Day originate?

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Feb. 14th is coming. To everyone's knowledge,it's the Valentine's Day. I think it is the most popular day of the world. At this day, most lovers will celebrate the romantic day together, those romantic men usually send roses or chocolate to their lovers.........However,how does this originate? And why would they rather to choose roses than any other flowers as the signal of their love? Anyone know?