Valentine's Day probably originated from the ancient myth that birds (usually lovebirds) began to mate on February 14th.
The 14th was also a feast day (until 1969, when it was dropped from the Roman Catholic calendar) to honor two Christian saints (one named Saint Valentine) sainted by the Roman Emperor Claudius II Gothicus. Saint Valentine was beheaded because he continued to marry young couples even though Claudius forbade it. Apparently Claudius thought that married soldiers weren't as good assingle soldiers.
In ancient Rome, February the 15th was the fertility festival (Lupercalia) in honor of the god Lupercus.
Through the centuries, it all merged together, and Saint Valentine's Day is a day dedicated to lovers, a time to exchange greeting cards and mushy words of never-ending LOVE.