The emotion of love is an affective emotion, directly reacting to goodness, rather than an aggressive one, reacting to challenge. Not only our so-called natural ability to grow and propagate exemplify natural love, but every faculty has a built-in affinity for what accords with its nature. By passion we mean some result of being acted on: either a form induced by the agent (like weight) or a movement consequent on the form (like falling to the ground). Whatever we desire acts on us in this way, first arousing an emotional attachment to itself and making itself agreeable, and then drawing us to seek it. The first change the object produces in our appetite is a feeling of its agreeableness: we call this love (weight can be thought of as a sort of natural love); then desire moves us to seek the object and pleasure comes to rest in it. Clearly then, as a change induced in us by an agent, love is a passion: the affective emotion strictly so, the will to love by stretching of the term. Love unites by making what is loved as agreeable to the lover as if it were himself or a part of himself. Though love is not itself a movement of the appetite towards an object, it is a change the appetite undergoes rendering an object agreeable. Favour is a freely chosen and willing love, open only to reasoning creatures; and charity―literally, holding dear―is a perfect form of love in which what is loved is highly prized. To love, as Aristotle says, is to want someone’s good; so its object is twofold: the good we want, loved with a love of desire, and the someone we want it for (ourselves or someone else), loved with a love of friendship. And just as what exist in the primary sense are subjects of existence, and properties exist only in a secondary sense, as modes in which subjects exist; so too what we love in the primary sense is the someone whose good we will, and only in a secondary sense do we love the good so willed. Friendship based on convenience or pleasure is friendship inasmuch as we want our friend’s good; but because this is subordinated to our own profit or pleasure such friendship is subordinated to love of desire and falls short of true friendship.
Thomas Aquinas, Summa Theologiae: A Concise Translation
love, philosophy, theology