It’s amusing to me when I come across answers to questions like that in philosophical works. People think that Aristotle, Aquinas, and Plato have nothing to say to them today, or that philosophy is dry as dust and lacks anything to do with “the real world,” and then we see something like this, where St Thomas explains what love actually is.
For it belongs properly to the nature of love that the lover wills the good of the beloved…True love requires one to will another’s good as one’s own. For a thing whose good one wills merely as conducive to another’s good, is loved accidentally: thus he who wills wine to be preserved that he may drink it, or who loves a man that he may be useful or pleasing to him, loves the wine or the man accidentally, but himself properly speaking. [SCG I:91 §§1–2]
Love is seeking the other person’s good as one’s own good. If we “love” another person for the sake of what he does for us, what we are actually doing is loving ourselves. It’s easy to see how this applies to most of what passes for “love:” infatuations are really nothing more than “loving” someone because of the feelings he (or she) inspires. It feels good to “love” that person, but that doesn’t mean that infatuation is actually love.
Aquinas, Etc. 
I find Aquinas to be exceptionally perspecacious on love and charity. It also, I think, helps to understand the Catholic doctrine of justification, since “making righteous” for Aquinas (and other Thomist Catholics and probably most Catholics?) simply means “made loving” (of God and people).
I actually posted my research so far on charity in Aquinas: http://theophilogue.files.wordpress.com/2012/01/love-and-charity-in-thomas-aquinas___last-edit_010612__.pdf
I’ve enjoyed reading several of your posts today.
Pax,
Bradley
Hi Bradley,
I’m sorry it has taken so long for me to respond. Thanks for your kind words.
I agree about St. Thomas’ clarity; I am very sure that when I don’t understand him the fault must be mine :-) His writing style fits my brain very well.
I’ll take a look at the link when I can.
Peace,
Fred