Objection 1. It would seem that the rewards of the beatitudes are unsuitably enumerated. Because the kingdom of heaven, which is eternal life, contains all good things. Therefore, once given the kingdom of heaven, no other rewards should be mentioned.
Objection 2. Further, the kingdom of heaven is assigned as the reward, both of the first and of the eighth beatitude. Therefore, on the same ground it should have been assigned to all.
Objection 3. Further, the beatitudes are arranged in the ascending order, as Augustine remarks (De Serm. Dom. in Monte i, 4): whereas the rewards seem to be placed in the descending order, since to "possess the land" is less than to possess "the kingdom of heaven." Therefore these rewards are unsuitably enumerated.
On the contrary, stands the authority of Our Lord Who propounded these rewards.
I answer that, These rewards are most suitably assigned, considering the nature of the beatitudes in relation to the three kinds of happiness indicated above (Article 3). For the first three beatitudes concerned the withdrawal of man from those things in which sensual happiness consists: which happiness man desires by seeking the object of his natural desire, not where he should seek it, viz. in God, but in temporal and perishable things. Wherefore the rewards of the first three beatitudes correspond to these things which some men seek to find in earthly happiness. For men seek in external things, viz. riches and honors, a certain excellence and abundance, both of which are implied in the kingdom of heaven, whereby man attains to excellence and abundance of good things in God. Hence Our Lord promised the kingdom of heaven to the poor in spirit. Again, cruel and pitiless men seek by wrangling and fighting to destroy their enemies so as to gain security for themselves. Hence Our Lord promised the meek a secure and peaceful possession of the land of the living, whereby the solid reality of eternal goods is denoted. Again, men seek consolation for the toils of the present life, in the lusts and pleasures of the world. Hence Our Lord promises comfort to those that mourn.
Two other beatitudes belong to the works of active happiness, which are the works of virtues directing man in his relations to his neighbor: from which operations some men withdraw through inordinate love of their own good. Hence Our Lord assigns to these beatitudes rewards in correspondence with the motives for which men recede from them. For there are some who recede from acts of justice, and instead of rendering what is due, lay hands on what is not theirs, that they may abound in temporal goods. Wherefore Our Lord promised those who hunger after justice, that they shall have their fill. Some, again, recede from works of mercy, lest they be busied with other people's misery. Hence Our Lord promised the merciful that they should obtain mercy, and be delivered from all misery.
The last two beatitudes belong to contemplative happiness or beatitude: hence the rewards are assigned in correspondence with the dispositions included in the merit. For cleanness of the eye disposes one to see clearly: hence the clean of heart are promised that they shall see God. Again, to make peace either in oneself or among others, shows a man to be a follower of God, Who is the God of unity and peace. Hence, as a reward, he is promised the glory of the Divine sonship, consisting in perfect union with God through consummate wisdom.
Reply to Objection 1. As Chrysostom says (Hom. xv in Matth.), all these rewards are one in reality, viz. eternal happiness, which the human intellect cannot grasp. Hence it was necessary to describe it by means of various boons known to us, while observing due proportion to the merits to which those rewards are assigned.
Reply to Objection 2. Just as the eighth beatitude is a confirmation of all the beatitudes, so it deserves all the rewards of the beatitudes. Hence it returns to the first, that we may understand all the other rewards to be attributed to it in consequence. Or else, according to Ambrose (Super Luc. v), the kingdom of heaven is promised to the poor in spirit, as regards the glory of the soul; but to those who suffer persecution in their bodies, it is promised as regards the glory of the body.
Reply to Objection 3. The rewards are also arranged in ascending order. For it is more to possess the land of the heavenly kingdom than simply to have it: since we have many things without possessing them firmly and peacefully. Again, it is more to be comforted in the kingdom than to have and possess it, for there are many things the possession of which is accompanied by sorrow. Again, it is more to have one's fill than simply to be comforted, because fulness implies abundance of comfort. And mercy surpasses satiety, for thereby man receives more than he merited or was able to desire. And yet more is it to see God, even as he is a greater man who not only dines at court, but also sees the king's countenance. Lastly, the highest place in the royal palace belongs to the king's son.
Source: Summa Theologica (New Advent)