Objection 1. It would seem that no precept should be given relating to the virtue of hope. For when an effect is sufficiently procured by one cause, there is no need to induce it by another. Now man is sufficiently induced by his natural inclination to hope for good. Therefore there is no need of a precept of the Law to induce him to do this.
Objection 2. Further, since precepts are given about acts of virtue, the chief precepts are about the acts of the chief virtues. Now the chief of all the virtues are the three theological virtues, viz. hope, faith and charity. Consequently, as the chief precepts of the Law are those of the decalogue, to which all others may be reduced, as stated above (I-II, 100, 3), it seems that if any precept of hope were given, it should be found among the precepts of the decalogue. But it is not to be found there. Therefore it seems that the Law should contain no precept of hope.
Objection 3. Further, to prescribe an act of virtue is equivalent to a prohibition of the act of the opposite vice. Now no precept is to be found forbidding despair which is contrary to hope. Therefore it seems that the Law should contain no precept of hope.
On the contrary, Augustine says on John 15:12, "This is My commandment, that you love one another" (Tract. lxxxiii in Joan.): "How many things are commanded us about faith! How many relating to hope!" Therefore it is fitting that some precepts should be given about hope.
I answer that, Among the precepts contained in Holy Writ, some belong to the substance of the Law, others are preambles to the Law. The preambles to the Law are those without which no law is possible: such are the precepts relating to the act of faith and the act of hope, because the act of faith inclines man's mind so that he believes the Author of the Law to be One to Whom he owes submission, while, by the hope of a reward, he is induced to observe the precepts. The precepts that belong to the substance of the Law are those which relate to right conduct and are imposed on man already subject and ready to obey: wherefore when the Law was given these precepts were set forth from the very outset under form of a command.
Yet the precepts of hope and faith were not to be given under the form of a command, since, unless man already believed and hoped, it would be useless to give him the Law: but, just as the precept of faith had to be given under the form of an announcement or reminder, as stated above (Question 16, Article 1), so too, the precept of hope, in the first promulgation of the Law, had to be given under the form of a promise. For he who promises rewards to them that obey him, by that very fact, urges them to hope: hence all the promises contained in the Law are incitements to hope.
Since, however, when once the Law has been given, it is for a wise man to induce men not only to observe the precepts, but also, and much more, to safeguard the foundation of the Law, therefore, after the first promulgation of the Law, Holy Writ holds out to man many inducements to hope, even by way of warning or command, and not merely by way of promise, as in the Law; for instance, in the Psalm 61:9: "Hope [Douay: 'Trust'] in Him all ye congregation of the people," and in many other passages of the Scriptures.
Reply to Objection 1. Nature inclines us to hope for the good which is proportionate to human nature; but for man to hope for a supernatural good he had to be induced by the authority of the Divine law, partly by promises, partly by admonitions and commands. Nevertheless there was need for precepts of the Divine law to be given even for those things to which natural reason inclines us, such as the acts of the moral virtues, for sake of insuring a greater stability, especially since the natural reason of man was clouded by the lusts of sin.
Reply to Objection 2. The precepts of the law of the decalogue belong to the first promulgation of the Law: hence there was no need for a precept of hope among the precepts of the decalogue, and it was enough to induce men to hope by the inclusion of certain promises, as in the case of the first and fourth commandments.
Reply to Objection 3. In those observances to which man is bound as under a duty, it is enough that he receive an affirmative precept as to what he has to do, wherein is implied the prohibition of what he must avoid doing: thus he is given a precept concerning the honor due to parents, but not a prohibition against dishonoring them, except by the law inflicting punishment on those who dishonor their parents. And since in order to be saved it is man's duty to hope in God, he had to be induced to do so by one of the above ways, affirmatively, so to speak, wherein is implied the prohibition of the opposite.
Source: Summa Theologica (New Advent)