Objection 1. It would seem that the craving for unity is not a cause of sorrow. For the Philosopher says (Ethic. x, 3) that "this opinion," which held repletion to be the cause of pleasure, and division [Aristotle wrote endeian, 'want'; St. Thomas, in the Latin version, read 'incisionem'; should he have read 'indigentiam'?], the cause of sorrow, "seems to have originated in pains and pleasures connected with food." But not every pleasure or sorrow is of this kind. Therefore the craving for unity is not the universal cause of sorrow; since repletion pertains to unity, and division is the cause of multitude.
Objection 2. Further, every separation is opposed to unity. If therefore sorrow were caused by a craving for unity, no separation would be pleasant: and this is clearly untrue as regards the separation of whatever is superfluous.
Objection 3. Further, for the same reason we desire the conjunction of good and the removal of evil. But as conjunction regards unity, since it is a kind of union; so separation is contrary to unity. Therefore the craving for unity should not be reckoned, rather than the craving for separation, as causing sorrow.
On the contrary, Augustine says (De Lib. Arb. iii, 23), that "from the pain that dumb animals feel, it is quite evident how their souls desire unity, in ruling and quickening their bodies. For what else is pain but a feeling of impatience of division or corruption?"
I answer that, Forasmuch as the desire or craving for good is reckoned as a cause of sorrow, so must a craving for unity, and love, be accounted as causing sorrow. Because the good of each thing consists in a certain unity, inasmuch as each thing has, united in itself, the elements of which its perfection consists: wherefore the Platonists held that "one" is a principle, just as "good" is. Hence everything naturally desires unity, just as it desires goodness: and therefore, just as love or desire for good is a cause of sorrow, so also is the love or craving for unity.
Reply to Objection 1. Not every kind of union causes perfect goodness, but only that on which the perfect being of a thing depends. Hence neither does the desire of any kind of unity cause pain or sorrow, as some have maintained: whose opinion is refuted by the Philosopher from the fact that repletion is not always pleasant; for instance, when a man has eaten to repletion, he takes no further pleasure in eating; because repletion or union of this kind, is repugnant rather than conducive to perfect being. Consequently sorrow is caused by the craving, not for any kind of unity, but for that unity in which the perfection of nature consists.
Reply to Objection 2. Separation can be pleasant, either because it removes something contrary to a thing's perfection, or because it has some union connected with it, such as union of the sense to its object.
Reply to Objection 3. Separation from things hurtful and corruptive is desired, in so far as they destroy the unity which is due. Wherefore the desire for such like separation is not the first cause of sorrow, whereas the craving for unity is.
Source: Summa Theologica (New Advent)