Someone said that you either love this day or hate it; there is no neutral ground.
Well, however you feel about this holiday, you can credit or blame this man:
Chaucer, the medieval English poet best know for The Canterbury Tales, wrote a poem entitled "The Parliment of Foules" (1382) which is based on the legend that February 14th is the day when the birds choose their mates for the year:
For this was on seynt Volantynys day
Whan euery bryd comyth there to chese [choose] his make [mate].
On this foundation was Hallmark's fortune laid.