Objection 1. It would seem that the lover does not do everything from love. For love is a passion, as stated above (Question 26, Article 2). But man does not do everything from passion: but some things he does from choice, and some things from ignorance, as stated in Ethic. v, 8. Therefore not everything that a man does, is done from love.
Objection 2. Further, the appetite is a principle of movement and action in all animals, as stated in De Anima iii, 10. If, therefore, whatever a man does is done from love, the other passions of the appetitive faculty are superfluous.
Objection 3. Further, nothing is produced at one and the same time by contrary causes. But some things are done from hatred. Therefore all things are not done from love.
On the contrary, Dionysius says (Div. Nom. iv) that "all things, whatever they do, they do for the love of good."
I answer that, Every agent acts for an end, as stated above (Question 1, Article 2). Now the end is the good desired and loved by each one. Wherefore it is evident that every agent, whatever it be, does every action from love of some kind.
Reply to Objection 1. This objection takes love as a passion existing in the sensitive appetite. But here we are speaking of love in a general sense, inasmuch as it includes intellectual, rational, animal, and natural love: for it is in this sense that Dionysius speaks of love in chapter iv of De Divinis Nominibus.
Reply to Objection 2. As stated above (05; 27, 4) desire, sadness and pleasure, and consequently all the other passions of the soul, result from love. Wherefore every act proceeds from any passion, proceeds also from love as from a first cause: and so the other passions, which are proximate causes, are not superfluous.
Reply to Objection 3. Hatred also is a result of love, as we shall state further on (29, 2).
Source: Summa Theologica (New Advent)