Objection 1. It would seem that it is more meritorious to love one's neighbor than to love God. For the more meritorious thing would seem to be what the Apostle preferred. Now the Apostle preferred the love of our neighbor to the love of God, according to Romans 9:3: "I wished myself to be an anathema from Christ for my brethren." Therefore it is more meritorious to love one's neighbor than to love God.
Objection 2. Further, in a certain sense it seems to be less meritorious to love one's friend, as stated above (Article 7). Now God is our chief friend, since "He hath first loved us" (1 John 4:10). Therefore it seems less meritorious to love God.
Objection 3. Further, whatever is more difficult seems to be more virtuous and meritorious since "virtue is about that which is difficult and good" (Ethic. ii, 3). Now it is easier to love God than to love one's neighbor, both because all things love God naturally, and because there is nothing unlovable in God, and this cannot be said of one's neighbor. Therefore it is more meritorious to love one's neighbor than to love God.
On the contrary, That on account of which a thing is such, is yet more so. Now the love of one's neighbor is not meritorious, except by reason of his being loved for God's sake. Therefore the love of God is more meritorious than the love of our neighbor.
I answer that, This comparison may be taken in two ways. First, by considering both loves separately: and then, without doubt, the love of God is the more meritorious, because a reward is due to it for its own sake, since the ultimate reward is the enjoyment of God, to Whom the movement of the Divine love tends: hence a reward is promised to him that loves God (John 14:21): "He that loveth Me, shall be loved of My Father, and I will... manifest Myself to him." Secondly, the comparison may be understood to be between the love of God alone on the one side, and the love of one's neighbor for God's sake, on the other. On this way love of our neighbor includes love of God, while love of God does not include love of our neighbor. Hence the comparison will be between perfect love of God, extending also to our neighbor, and inadequate and imperfect love of God, for "this commandment we have from God, that he, who loveth God, love also his brother" (1 John 4:21).
Reply to Objection 1. According to one gloss, the Apostle did not desire this, viz. to be severed from Christ for his brethren, when he was in a state of grace, but had formerly desired it when he was in a state of unbelief, so that we should not imitate him in this respect.
We may also reply, with Chrysostom (De Compunct. i, 8) [Hom. xvi in Ep. ad Rom.] that this does not prove the Apostle to have loved his neighbor more than God, but that he loved God more than himself. For he wished to be deprived for a time of the Divine fruition which pertains to love of one self, in order that God might be honored in his neighbor, which pertains to the love of God.
Reply to Objection 2. A man's love for his friends is sometimes less meritorious in so far as he loves them for their sake, so as to fall short of the true reason for the friendship of charity, which is God. Hence that God be loved for His own sake does not diminish the merit, but is the entire reason for merit.
Reply to Objection 3. The "good" has, more than the "difficult," to do with the reason of merit and virtue. Therefore it does not follow that whatever is more difficult is more meritorious, but only what is more difficult, and at the same time better.
Source: Summa Theologica (New Advent)