I'm posting this as a kind of follow-up to the Viking's talk, though I wrote it long ago (when I was trying to persuade him to give the talk).
One of the first philosophical facts I learned about love, from something my mom said, is that love is to the will what a "light bulb moment" is to the intellect, because the good and the truth are both from God. The truth is correspondence with what is, and good is correspondence with He who Is. They are the ultimate focus of the intellect and the will, respectively, which is why St. Augustine says that we are restless until we rest in Him who is truth and goodness itself.
One implication that I drew from this correspondence of love with a "lightbulb moment," or understanding, is that, though love is a movement of the will, it is often involuntary--we can no more help loving something we perceive as good than we can help that moment of delight when we grasp a concept for the first time. Our will is made to seek the good, so on finding something good it turns towards it automatically, as for example most people's reaction to ice cream.
This applies to all or most meanings of the word love; one of the things you can learn from Tolkien the philologist is that when the same or a related word is used for concepts that seem different to us, it usually means that at some distant time and place they were connected in meaning as well as in name. Thus loving ice cream, loving your husband, and loving your enemy are all at bottom the same thing, though they work in different ways. When you taste ice cream, for example, your taste buds tell you "this is good," and your will says, "well, then I want it," and that is loving ice cream. You love a person when your will is moved towards the good you find in them.
The difference between the different types of love lies in the degrees of goodness in the object and the clarity of our perception of this goodness. God can have perfect love for even an imperfect creature, seeing all the good in it because He is the source of all good, but the kind of love anyone, even God, can have for ice cream is limited by the nature of ice cream, which is good in only a very limited way--in creaminess, sweetness, freshness--as opposed to a person who can be good by heroic faith, hope, and charity.
Another limit in our capacity to love is that we can only love what we know, and often our knowledge of a thing or person is limited or faulty, as happens to Elizabeth in Pride and Prejudice. Her prejudice blinds her to Darcy's good qualities and hinders her from loving him, even though they are the very qualities that she loves best. The difficulty of loving your enemies is that you have to move your will (often almost by force) towards the good God sees in them, for the very fact that He made them who can make no evil, rather than for any good that you personally can see in them.
In the case of "falling in love," on the other hand, the outstanding thing is that you perceive great good in the person without much effort in looking for it--and the movement of the will is instantaneous and effortless. For example, I easily recognize the good of a keen intellect and a certain type of humor, and would be likely to fall in love with someone who has these qualities, whereas other people might notice more someone's humility, mechanical skill, or other good characteristics. Your "soulmate" is the one who most incarnates the goods that you most clearly perceive, and vice versa--someone in whom you can always discover, and help to polish, new facets of good, reflections of different aspects of the infinite goodness of God.
Of course, love can also be a voluntary act in the ordinary sense. You can look for and love the good in anyone, and it would be easier to find it in a spouse because of the shared life of marriage, which places you in the best position to look for it--this is why arranged marriages can work. But I think it is ALSO possible for true love at first sight to happen--a person's face, voice, and gestures can tell us a lot about them, even enough to move a true and lasting love. The most important thing to remember, I guess, is that God is love, and ultimate love can only be found with and through Him.