Objection 1. It seems that the fourth precept, about honoring one's parents, is unfittingly expressed. For this is the precept pertaining to piety. Now, just as piety is a part of justice, so are observance, gratitude, and others of which we have spoken (Q101,102, seq.). Therefore it seems that there should not have been given a special precept of piety, as none is given regarding the others.
Objection 2. Further, piety pays worship not only to one's parents, but also to one's country, and also to other blood kindred, and to the well-wishers of our country, as stated above (101, 1 and 2). Therefore it was unfitting for this precept to mention only the honoring of one's father and mother.
Objection 3. Further, we owe our parents not merely honor but also support. Therefore the mere honoring of one's parents is unfittingly prescribed.
Objection 4. Further, sometimes those who honor their parents die young, and on the contrary those who honor them not live a long time. Therefore it was unfitting to supplement this precept with the promise, "That thou mayest be long-lived upon earth."
On the contrary, stands the authority of Scripture.
I answer that, The precepts of the decalogue are directed to the love of God and of our neighbor. Now to our parents, of all our neighbors, we are under the greatest obligation. Hence, immediately after the precepts directing us to God, a place is given to the precept directing us to our parents, who are the particular principle of our being, just as God is the universal principle: so that this precept has a certain affinity to the precepts of the First Table.
Reply to Objection 1. As stated above (Question 101, Article 2), piety directs us to pay the debt due to our parents, a debt which is common to all. Hence, since the precepts of the decalogue are general precepts, they ought to contain some reference to piety rather than to the other parts of justice, which regard some special debt.
Reply to Objection 2. The debt to one's parents precedes the debt to one's kindred and country since it is because we are born of our parents that our kindred and country belong to us. Hence, since the precepts of the decalogue are the first precepts of the Law, they direct man to his parents rather than to his country and other kindred. Nevertheless this precept of honoring our parents is understood to command whatever concerns the payment of debt to any person, as secondary matter included in the principal matter.
Reply to Objection 3. Reverential honor is due to one's parents as such, whereas support and so forth are due to them accidentally, for instance, because they are in want, in slavery, or the like, as stated above (Question 101, Article 2). And since that which belongs to a thing by nature precedes that which is accidental, it follows that among the first precepts of the Law, which are the precepts of the decalogue, there is a special precept of honoring our parents: and this honor, as a kind of principle, is understood to comprise support and whatever else is due to our parents.
Reply to Objection 4. A long life is promised to those who honor their parents not only as to the life to come, but also as to the present life, according to the saying of the Apostle (1 Timothy 4:8): "Piety [Douay: 'godliness'] is profitable to all things, having promise of the life that now is and of that which is to come." And with reason. Because the man who is grateful for a favor deserves, with a certain congruity, that the favor should be continued to him, and he who is ungrateful for a favor deserves to lose it. Now we owe the favor of bodily life to our parents after God: wherefore he that honors his parents deserves the prolongation of his life, because he is grateful for that favor: while he that honors not his parents deserves to be deprived of life because he is ungrateful for the favor. However, present goods or evils are not the subject of merit or demerit except in so far as they are directed to a future reward, as stated above (I-II, 114, 12). Wherefore sometimes in accordance with the hidden design of the Divine judgments, which regard chiefly the future reward, some, who are dutiful to their parents, are sooner deprived of life, while others, who are undutiful to their parents, live longer.
Source: Summa Theologica (New Advent)