The ancient Greeks had numerous conceptions of the word "love." Eros (cf. "erotic") is probably the one most associated with Valentine's Day: the sensual, passionate love between lovers, especially of a budding romance. But even that elderly, sometimes cranky but nevertheless cute couple you see at the grocery store heatedly debating what kind of milk to buy possesses eros. As relationships mature and strengthen, eros naturally becomes less physical and more emotional. That happens when one falls in love with the true essence of who someone is--when lovers bring out the best in one another and appreciate each other's beauty from deep within.
Philia is the kind of love between good friends. It requires some degree of familiarity, and usually a mutual desire to partake in some kind of enjoyable activity together. The scene in the blockbuster Superbad when Seth gently taps Evan's nose while confessing his love readily comes to mind, lol. He really did pose a good question though--why DON'T good friends say that more often? Ah well, must be a cultural thing.
And finally, agape is the brand of love which I understand least, but perhaps need most to become intimate with. Agape love is a generalized affection toward anyone whom you hold in high regard for no particular reason. Love is not elicited by some lovable quality about the object of one's love; it is simply loving for loving's sake. The Latin derivative is caritas/caritatis, which means charity. Often alluded to in Christian texts, agape is an unconditional, selfless, self-sacrificial, or charitable variety of love which knows no boundaries and is to be extended to both friends and enemies alike. An appropriate scripture would be, "Love thy neighbor as you love thyself." Agape is at work when you volunteer your time to feed the homeless at a food kitchen, or even when you simply forgive someone who has wronged you.
I think it's interesting that the Greeks were so discerning about this ever so human quality. (Hmm...it's probably not exclusive to our species though.) But I like that linguistic evolution has merged these separate conceptions into one expression. In the Romance languages, it's "amor," or something very close to that. In English, we just know it as "love." But what is it, exactly? We can't really point to it. I guess we just know it when we feel it. It's the lovey-dovey, bubbly feeling we get inside, that "aww" factor that makes us melt. I think scientists call it oxytocin, the "hug" or "cuddle" hormone. To you, the Reader, allow me to express some agape and extend a virtual "hug." So, *big hug* lol.
Today, in the spirit of St. Valentine's Day, tell and/or show at least one person "I love you." 
