Objection 1. It would seem that some created good constitutes man's happiness. For Dionysius says (Div. Nom. vii) that Divine wisdom "unites the ends of first things to the beginnings of second things," from which we may gather that the summit of a lower nature touches the base of the higher nature. But man's highest good is happiness. Since then the angel is above man in the order of nature, as stated in I, 111, 1, it seems that man's happiness consists in man somehow reaching the angel.
Objection 2. Further, the last end of each thing is that which, in relation to it, is perfect: hence the part is for the whole, as for its end. But the universe of creatures which is called the macrocosm, is compared to man who is called the microcosm (Phys. viii, 2), as perfect to imperfect. Therefore man's happiness consists in the whole universe of creatures.
Objection 3. Further, man is made happy by that which lulls his natural desire. But man's natural desire does not reach out to a good surpassing his capacity. Since then man's capacity does not include that good which surpasses the limits of all creation, it seems that man can be made happy by some created good. Consequently some created good constitutes man's happiness.
On the contrary, Augustine says (De Civ. Dei xix, 26): "As the soul is the life of the body, so God is man's life of happiness: of Whom it is written: 'Happy is that people whose God is the Lord' (Psalm 143:15)."
I answer that, It is impossible for any created good to constitute man's happiness. For happiness is the perfect good, which lulls the appetite altogether; else it would not be the last end, if something yet remained to be desired. Now the object of the will, i.e. of man's appetite, is the universal good; just as the object of the intellect is the universal true. Hence it is evident that naught can lull man's will, save the universal good. This is to be found, not in any creature, but in God alone; because every creature has goodness by participation. Wherefore God alone can satisfy the will of man, according to the words of Psalm 102:5: "Who satisfieth thy desire with good things." Therefore God alone constitutes man's happiness.
Reply to Objection 1. The summit of man does indeed touch the base of the angelic nature, by a kind of likeness; but man does not rest there as in his last end, but reaches out to the universal fount itself of good, which is the common object of happiness of all the blessed, as being the infinite and perfect good.
Reply to Objection 2. If a whole be not the last end, but ordained to a further end, then the last end of a part thereof is not the whole itself, but something else. Now the universe of creatures, to which man is compared as part to whole, is not the last end, but is ordained to God, as to its last end. Therefore the last end of man is not the good of the universe, but God himself.
Reply to Objection 3. Created good is not less than that good of which man is capable, as of something intrinsic and inherent to him: but it is less than the good of which he is capable, as of an object, and which is infinite. And the participated good which is in an angel, and in the whole universe, is a finite and restricted good.
Source: Summa Theologica (New Advent)