The Catholic Encyclopedia has a bit of history and gives some insight into capitalisation:
The popular customs associated with Saint Valentine's Day undoubtedly had their origin in a conventional belief generally received in England and France during the Middle Ages, that on 14 February, i.e. half way through the second month of the year, the birds began to pair. Thus in Chaucer's Parliament of Foules we read:
For this was sent on Seynt Valentyne's day
Whan every foul cometh ther to choose his mate.
For this reason the day was looked upon as specially consecrated to lovers and as a proper occasion for writing love letters and sending lovers' tokens. [...]
In the Paston Letters, Dame Elizabeth Brews writes thus about a match she hopes to make for her daughter (we modernize the spelling), addressing the favoured suitor:
And, cousin mine, upon Monday is Saint Valentine's Day and every bird chooses himself a mate, and if it like you to come on Thursday night, and make provision that you may abide till then, I trust to God that ye shall speak to my husband and I shall pray that we may bring the matter to a conclusion.
Shortly after the young lady herself wrote a letter to the same man addressing it "Unto my rightwell beloved Valentine, John Paston Esquire". The custom of choosing and sending valentines has of late years fallen into comparative desuetude.
The token of love (a card, gift) is a valentine. It's a common noun and doesn't merit a capital letter. Calling someone beloved a Valentine is an example of metonymy — giving them the name of a saint as a metaphor. Being a name, it is a proper noun and does deserve a capital letter.
[In this case, it's rather an extended metaphor. Using "The White House" as a metonym for the President's administration is simple. Here, Valentine refers to the saint and his day, and using that date as significant in getting a mate. But you're still recalling a person or renaming your lover, "Be my Valentine".]