Because that, when they knew God, they glorified him not as God, neither were thankful; but became vain in their imaginations, and their foolish heart was darkened. Romans 1:21
"Tradition
dictated that priests would gather at the entrance to the 
sacred cave
where Romulus and Remus were believed to have been nurtured by a
she-wolf.
They would then sacrifice a goat (for fertility) and a dog
(for purification).
The
goat's hide would be cut into strips and dipped into sacrificial blood,
with priests taking the bloody skin chunks to the streets and literally
slapping Roman women of childbearing age with them in the belief that
it would increase the likelihood of them becoming pregnant.
The story
continues that, at the end of the holiday, those women who were slapped
would place bits of papyrus with their names written on them in a big
urn, and the city's biologically-viable bachelors would each choose a
name, becoming "coupled" with her for one year – which would sometimes
end in marriage if things went well.
Legends
about a Valentine -- say that after a Roman emperor prohibited marriage for
young men due to a belief that single men were better warriors than
those with families. The story goes that Valentine refused to obey and
helped other young soldiers to be married. When our purported Valentine
was found out, the emperor reportedly sentenced him to death.
Other
stories claim that an imprisoned Valentine sent the first "valentine"
to a young woman he was enamored with (some suggest that it was the
jailor’s daughter), before being executed.
At the end of the 5th century, however, Pope Gelasius I declared 14
February to be St. Valentine’s Day, a move to "Christianize" a fertility
festival without cancelling it completely." Sputnik
