Objection 1. It would seem that desire is not a cause of pain or sorrow. Because sorrow of itself regards evil, as stated above (Article 1): whereas desire is a movement of the appetite towards good. Now movement towards one contrary is not a cause of movement towards the other contrary. Therefore desire is not a cause of pain.
Objection 2. Further, pain, according to Damascene (De Fide Orth. ii, 12), is caused by something present; whereas the object of desire is something future. Therefore desire is not a cause of pain.
Objection 3. Further, that which is pleasant in itself is not a cause of pain. But desire is pleasant in itself, as the Philosopher says (Rhet. i, 11). Therefore desire is not a cause of pain or sorrow.
On the contrary, Augustine says (Enchiridion xxiv): "When ignorance of things necessary to be done, and desire of things hurtful, found their way in: error and pain stole an entrance in their company." But ignorance is the cause of error. Therefore desire is a cause of sorrow.
I answer that, Sorrow is a movement of the animal appetite. Now, as stated above (Article 1), the appetitive movement is likened to the natural appetite; a likeness, that may be assigned to a twofold cause; one, on the part of the end, the other, on the part of the principle of movement. Thus, on the part of the end, the cause of a heavy body's downward movement is the lower place; while the principle of that movement is a natural inclination resulting from gravity.
Now the cause of the appetitive movement, on the part of the end, is the object of that movement. And thus, it has been said above (Article 1) that the cause of pain or sorrow is a present evil. On the other hand, the cause, by way or principle, of that movement, is the inward inclination of the appetite; which inclination regards, first of all, the good, and in consequence, the rejection of a contrary evil. Hence the first principle of this appetitive movement is love, which is the first inclination of the appetite towards the possession of good: while the second principle is hatred, which is the first inclination of the appetite towards the avoidance of evil. But since concupiscence or desire is the first effect of love, which gives rise to the greatest pleasure, as stated above (Question 32, Article 6); hence it is that Augustine often speaks of desire or concupiscence in the sense of love, as was also stated (30, 2, ad 2): and in this sense he says that desire is the universal cause of sorrow. Sometimes, however, desire taken in its proper sense, is the cause of sorrow. Because whatever hinders a movement from reaching its end is contrary to that movement. Now that which is contrary to the movement of the appetite, is a cause of sorrow. Consequently, desire becomes a cause of sorrow, in so far as we sorrow for the delay of a desired good, or for its entire removal. But it cannot be a universal cause of sorrow: since we sorrow more for the loss of present good, in which we have already taken pleasure, than for the withdrawal of future good which we desire to have.
Reply to Objection 1. The inclination of the appetite to the possession of good causes the inclination of the appetite to fly from evil, as stated above. And hence it is that the appetitive movements that regard good, are reckoned as causing the appetitive movements that regard evil.
Reply to Objection 2. That which is desired, though really future, is, nevertheless, in a way, present, inasmuch as it is hoped for. Or we may say that although the desired good itself is future, yet the hindrance is reckoned as present, and so gives rise to sorrow.
Reply to Objection 3. Desire gives pleasure, so long as there is hope of obtaining that which is desired. But, when hope is removed through the presence of an obstacle, desire causes sorrow.
Source: Summa Theologica (New Advent)