The following is a repost from 2008.
You are struck by a vision, some exquisite example of beauty. Being struck, you are changed, this beautiful object introducing itself into your very disposition, so that you become, so to speak, adapted to it, so as to find satisfaction in it. You are pleased by it, and, being pleased by it, you desire it, and this desire seeks the joy and rest of its presence. Thus you have become caught up in a sort of circle: it has joined itself to you, by changing you; you are thereby driven to join yourself to it, that you may rejoice in it. You are set in motion by it, and this motion comes to rest only in that which started the motion in the first place.
Such is Thomas Aquinas's view of the passion of love. In this account, the experience of love consists in a series of changes induced in us, immutationes, the first of which, complacentia, the taking pleasure in, or being pleased by, a thing, is what we most often refer to as 'love'. The beloved becomes, in a sense, a part of the lover. But this complacentia isn't the term of the change; it continues on to desiderium, desire, the drive to union (of some sort) with what is loved, and the change involved in this desire continues until one finds a way to be united to what is loved, and rest in it. This rest is gaudium, joy.
This is all on the supposition that everything else is equal, of course; any discussion of the changes involved in the passions has a mercurial and unstable subject. There are endless numbers of things that might intervene. But there is enough pattern to the chaos that each of these, the amor or complacentia, the desiderium, the gaudium, is a recognizable feature, as is the sense of coaptatio, adaptation to the beloved, the experience of being disposed in some way by the loved one to love the loved one.
There is much more to St. Thomas's account of the passion than this; his discussion of the effects of love is particularly interesting. Love, says Thomas, has four proximate effects: liquefactio, fruitio, languor, fervor. In liquefactio our defenses are melted, our heart is softened; to the extent the beloved is present to us, we have fruitio, enjoyment; to the extent the beloved is absent, we have languor, sorrow or pining, and fervor, the passion to possess. These effects are induced in us proportional to the severity of the immutatio. Beyond this there are other effects that may ensue: union, indwelling (dwelling upon the beloved in thought and in sympathy), zeal or jealousy (understood as the repulsing of what stands in love's way), ecstasy (in the sense of being somehow carried away, either elevated beyond or debased below our usual state of sanity), and the myriad acts of lovers.