Objection 1. It would seem that a man is not bound to love his neighbor more than his own body. For his neighbor includes his neighbor's body. If therefore a man ought to love his neighbor more than his own body, it follows that he ought to love his neighbor's body more than his own.
Objection 2. Further, a man ought to love his own soul more than his neighbor's, as stated above (Article 4). Now a man's own body is nearer to his soul than his neighbor. Therefore we ought to love our body more than our neighbor.
Objection 3. Further, a man imperils that which he loves less for the sake of what he loves more. Now every man is not bound to imperil his own body for his neighbor's safety: this belongs to the perfect, according to John 15:13: "Greater love than this no man hath, that a man lay down his life for his friends." Therefore a man is not bound, out of charity, to love his neighbor more than his own body.
On the contrary, Augustine says (De Doctr. Christ. i, 27) that "we ought to love our neighbor more than our own body."
I answer that, Out of charity we ought to love more that which has more fully the reason for being loved out of charity, as stated above (2; 25, 12). Now fellowship in the full participation of happiness which is the reason for loving one's neighbor, is a greater reason for loving, than the participation of happiness by way of overflow, which is the reason for loving one's own body. Therefore, as regards the welfare of the soul we ought to love our neighbor more than our own body.
Reply to Objection 1. According to the Philosopher (Ethic. ix, 8) a thing seems to be that which is predominant in it: so that when we say that we ought to love our neighbor more than our own body, this refers to his soul, which is his predominant part.
Reply to Objection 2. Our body is nearer to our soul than our neighbor, as regards the constitution of our own nature: but as regards the participation of happiness, our neighbor's soul is more closely associated with our own soul, than even our own body is.
Reply to Objection 3. Every man is immediately concerned with the care of his own body, but not with his neighbor's welfare, except perhaps in cases of urgency: wherefore charity does not necessarily require a man to imperil his own body for his neighbor's welfare, except in a case where he is under obligation to do so and if a man of his own accord offer himself for that purpose, this belongs to the perfection of charity.
Source: Summa Theologica (New Advent)