The origins of St Valentine and his connection to Valentine's Day have long been disputed. There are many theories and assumptions surrounding St Valentine.
His connection to love and romance appears to stem from AD 270 when, as a priest, "he helped persecuted Christians," and he continued to perform marriages for couples even though the Roman emperor had prohibited marriage as he needed more soldiers for his army. When his practices were discovered he was imprisoned and scheduled for execution.
Valentine sent a note to a girl he had befriended, asking her to remain true to God, and he signed it from "your Valentine."
Valentine's efforts and his sacrifices were all in the name of the Church - it would have been essential to the faith for couples to marry so his efforts made him a martyr - anyone who dies in the protection and pursuance of his or her faith. Making him a saint would have followed through a process of declarations and a final decree as, in ancient times there were no official steps to canonization and subsequent sainthood.
In the Catholic Church, a Patron saint is one who will protect you in specific circumstances. St Anthony is the patron saint of lost things. St Christopher is the patron saint of travel and travellers and so on; hence, it seemed fitting to make St Valentine the patron saint of love as he spent his priesthood protecting and fighting for the sanctity of marriage.
All the saints have what are called "feast days," most of which we are oblivious to. One everybody knows is St Patrick's feast day (March 17) which has many Irish connotations but not any real religious reference anymore. So too with St Valentine. We still celebrate his feast day on February 14 but any religious significance is not considered important.