Objection 1. It would seem that sorrow is not most harmful to the body. For sorrow has a spiritual existence in the soul. But those things which have only a spiritual existence do not cause a transmutation in the body: as is evident with regard to the images of colors, which images are in the air and do not give color to bodies. Therefore sorrow is not harmful to the body.
Objection 2. Further if it be harmful to the body, this can only be due to its having a bodily transmutation in conjunction with it. But bodily transmutation takes place in all the passions of the soul, as stated above (22, 1,3). Therefore sorrow is not more harmful to the body than the other passions of the soul.
Objection 3. Further, the Philosopher says (Ethic. vii, 3) that "anger and desire drive some to madness": which seems to be a very great harm, since reason is the most excellent thing in man. Moreover, despair seems to be more harmful than sorrow; for it is the cause of sorrow. Therefore sorrow is not more harmful to the body than the other passions of the soul.
On the contrary, It is written (Proverbs 17:22): "A joyful mind maketh age flourishing: a sorrowful spirit drieth up the bones": and (Proverbs 25:20): "As a moth doth by a garment, and a worm by the wood: so the sadness of a man consumeth the heart": and (Sirach 38:19): "Of sadness cometh death."
I answer that, Of all the soul's passions, sorrow is most harmful to the body. The reason of this is because sorrow is repugnant to man's life in respect of the species of its movement, and not merely in respect of its measure or quantity, as is the case with the other passions of the soul. For man's life consists in a certain movement, which flows from the heart to the other parts of the body: and this movement is befitting to human nature according to a certain fixed measure. Consequently if this movement goes beyond the right measure, it will be repugnant to man's life in respect of the measure of quantity; but not in respect of its specific character: whereas if this movement be hindered in its progress, it will be repugnant to life in respect of its species.
Now it must be noted that, in all the passions of the soul, the bodily transmutation which is their material element, is in conformity with and in proportion to the appetitive movement, which is the formal element: just as in everything matter is proportionate to form. Consequently those passions that imply a movement of the appetite in pursuit of something, are not repugnant to the vital movement as regards its species, but they may be repugnant thereto as regards its measure: such are love, joy, desire and the like; wherefore these passions conduce to the well-being of the body; though, if they be excessive, they may be harmful to it. On the other hand, those passions which denote in the appetite a movement of flight or contraction, are repugnant to the vital movement, not only as regards its measure, but also as regards its species; wherefore they are simply harmful: such are fear and despair, and above all sorrow which depresses the soul by reason of a present evil, which makes a stronger impression than future evil.
Reply to Objection 1. Since the soul naturally moves the body, the spiritual movement of the soul is naturally the cause of bodily transmutation. Nor is there any parallel with spiritual images, because they are not naturally ordained to move such other bodies as are not naturally moved by the soul.
Reply to Objection 2. Other passions imply a bodily transmutation which is specifically in conformity with the vital movement: whereas sorrow implies a transmutation that is repugnant thereto, as stated above.
Reply to Objection 3. A lesser cause suffices to hinder the use of reason, than to destroy life: since we observe that many ailments deprive one of the use of reason, before depriving one of life. Nevertheless fear and anger cause very great harm to the body, by reason of the sorrow which they imply, and which arises from the absence of the thing desired. Moreover sorrow too sometimes deprives man of the use of reason: as may be seen in those who through sorrow become a prey to melancholy or madness.
Source: Summa Theologica (New Advent)