Objection 1. It would seem that men are not guarded by the angels. For guardians are deputed to some because they either know not how, or are not able, to guard themselves, as children and the sick. But man is able to guard himself by his free-will; and knows how by his natural knowledge of natural law. Therefore man is not guarded by an angel.
Objection 2. Further, a strong guard makes a weaker one superfluous. But men are guarded by God, according to Psalm 120:4: "He shall neither slumber nor sleep, that keepeth Israel." Therefore man does not need to be guarded by an angel.
Objection 3. Further, the loss of the guarded redounds to the negligence of the guardian; hence it was said to a certain one: "Keep this man; and if he shall slip away, thy life shall be for his life" (1 Kings 20:39). Now many perish daily through falling into sin; whom the angels could help by visible appearance, or by miracles, or in some such-like way. The angels would therefore be negligent if men are given to their guardianship. But that is clearly false. Therefore the angels are not the guardians of men.
On the contrary, It is written (Psalm 90:11): "He hath given His angels charge over thee, to keep thee in all thy ways."
I answer that, According to the plan of Divine Providence, we find that in all things the movable and variable are moved and regulated by the immovable and invariable; as all corporeal things by immovable spiritual substances, and the inferior bodies by the superior which are invariable in substance. We ourselves also are regulated as regards conclusions, about which we may have various opinions, by the principles which we hold in an invariable manner. It is moreover manifest that as regards things to be done human knowledge and affection can vary and fail from good in many ways; and so it was necessary that angels should be deputed for the guardianship of men, in order to regulate them and move them to good.
Reply to Objection 1. By free-will man can avoid evil to a certain degree, but not in any sufficient degree; forasmuch as he is weak in affection towards good on account of the manifold passions of the soul. Likewise universal natural knowledge of the law, which by nature belongs to man, to a certain degree directs man to good, but not in a sufficient degree; because in the application of the universal principles of law to particular actions man happens to be deficient in many ways. Hence it is written (Wisdom 9:14): "The thoughts of mortal men are fearful, and our counsels uncertain." Thus man needs to be guarded by the angels.
Reply to Objection 2. Two things are required for a good action; first, that the affection be inclined to good, which is effected in us by the habit of moral virtue.
Secondly, that reason should discover the proper methods to make perfect the good of virtue; this the Philosopher (Ethic. vi) attributes to prudence. As regards the first, God guards man immediately by infusing into him grace and virtues; as regards the second, God guards man as his universal instructor, Whose precepts reach man by the medium of the angels, as above stated (111, 1).
Reply to Objection 3. As men depart from the natural instinct of good by reason of a sinful passion, so also do they depart from the instigation of the good angels, which takes place invisibly when they enlighten man that he may do what is right. Hence that men perish is not to be imputed to the negligence of the angels but to the malice of men. That they sometimes appear to men visibly outside the ordinary course of nature comes from a special grace of God, as likewise that miracles occur outside the order of nature.
Source: Summa Theologica (New Advent)