Objection 1. It would seem that this sacrament should be conferred even on those who are in good health. For the healing of the mind is a more important effect of this sacrament than the healing of the body, as stated above (Question 30, Article 2). Now even those who are healthy in body need to be healed in mind. Therefore this sacrament should be conferred on them also.
Objection 2. Further, this is the sacrament of those who are departing this life, just as Baptism is the sacrament of those who are entering this life. Now Baptism is given to all who enter. Therefore this sacrament should be given to all who are departing. But sometimes those who are near departure are in good health, for instance those who are to be beheaded. Therefore this sacrament should be conferred on them.
On the contrary, It is written (James 5:14): "Is any man sick among you," etc. Therefore none but the sick are competent to receive this sacrament.
I answer that, This sacrament is a spiritual healing, as stated above (30, 1,2), and is signified by way of a healing of the body. Hence this sacrament should not be conferred on those who are not subjects for bodily healing, those namely, who are in good health.
Reply to Objection 1. Although spiritual health is the principal effect of this sacrament, yet this same spiritual healing needs to be signified by a healing of the body, although bodily health may not actually ensue. Consequently spiritual health can be conferred by this sacrament on those alone who are competent to receive bodily healing, viz. the sick; even as he alone can receive Baptism who is capable of a bodily washing, and not a child yet in its mother's womb.
Reply to Objection 2. Even those who are entering into life cannot receive Baptism unless they are capable of a bodily washing. And so those who are departing this life cannot receive this sacrament, unless they be subjects for a bodily healing.
Source: Summa Theologica (New Advent)