Lol, paganism? Valentine's Day is a Christian holiday that is based on a Catholic saint's feast day, St. Valentine, who was a 3rd century martyr that came to be associated with courtly love in the middle ages.
A wealthy Roman Christian named Valentinus was martyred and buried near Rome, he was included in Eusebius of Caesarea's accounts of martyrs and thus immortalized for Christendom along with quite a few others, because this particular Eusebius also wrote the most widely studied and ergo most important general history of the Church from the first four centuries.
Most of Christendom was Roman Catholic for quite some time, and people in the middle ages slapped all kinds of stories and themes onto random martyrs, of whom all they really could know was the sole fact that they were martyrs. So presto you have this guy become associated with the weird social rules for people having feelings for each other at the ruler's court, eventually that theme gets fantasized further by the conception of King Arthur and the Knights of the Round Table, and now here we are with a St. Valentine who just has to do with couples in general.
So... pretty much the same as lots of other Catholic holidays, like St. Patrick's Day and the story about the snakes.