Zenit has an interview with Catholic University's Monsignor Sokolowski about the Pope's encyclical ... it includes this interesting tidbit (among some others):

Q: Where does love fit into the philosophical discipline of natural theology, or any other area of philosophy where the question of God is addressed?

Monsignor Sokolowski: Love has been a theme in philosophy from the beginning; think of the role it has in Plato's dialogues. When Aristotle speaks about human happiness, he discusses various ways in which we desire and wish for things. Even the theoretic life is a good for us that we can love. Lucretius begins "De Rerum Natura" with a hymn to "alma Venus," who pervades and governs everything. Philosophy gets to ultimates and the good is among them, so our response to the good -- love -- is among the ultimates as well.