Objection 1. It would seem that happiness consists in power. For all things desire to become like to God, as to their last end and first beginning. But men who are in power, seem, on account of the similarity of power, to be most like to God: hence also in Scripture they are called "gods" (Exodus 22:28), "Thou shalt not speak ill of the gods." Therefore happiness consists in power.
Objection 2. Further, happiness is the perfect good. But the highest perfection for man is to be able to rule others; which belongs to those who are in power. Therefore happiness consists in power.
Objection 3. Further, since happiness is supremely desirable, it is contrary to that which is before all to be shunned. But, more than aught else, men shun servitude, which is contrary to power. Therefore happiness consists in power.
On the contrary, Happiness is the perfect good. But power is most imperfect. For as Boethius says (De Consol. iii), "the power of man cannot relieve the gnawings of care, nor can it avoid the thorny path of anxiety": and further on: "Think you a man is powerful who is surrounded by attendants, whom he inspires with fear indeed, but whom he fears still more?"
I answer that, It is impossible for happiness to consist in power; and this for two reasons. First because power has the nature of principle, as is stated in Metaph. v, 12, whereas happiness has the nature of last end. Secondly, because power has relation to good and evil: whereas happiness is man's proper and perfect good. Wherefore some happiness might consist in the good use of power, which is by virtue, rather than in power itself.
Now four general reasons may be given to prove that happiness consists in none of the foregoing external goods. First, because, since happiness is man's supreme good, it is incompatible with any evil. Now all the foregoing can be found both in good and in evil men. Secondly, because, since it is the nature of happiness to "satisfy of itself," as stated in Ethic. i, 7, having gained happiness, man cannot lack any needful good. But after acquiring any one of the foregoing, man may still lack many goods that are necessary to him; for instance, wisdom, bodily health, and such like. Thirdly, because, since happiness is the perfect good, no evil can accrue to anyone therefrom. This cannot be said of the foregoing: for it is written (Ecclesiastes 5:12) that "riches" are sometimes "kept to the hurt of the owner"; and the same may be said of the other three. Fourthly, because man is ordained to happiness through principles that are in him; since he is ordained thereto naturally. Now the four goods mentioned above are due rather to external causes, and in most cases to fortune; for which reason they are called goods of fortune. Therefore it is evident that happiness nowise consists in the foregoing.
Reply to Objection 1. God's power is His goodness: hence He cannot use His power otherwise than well. But it is not so with men. Consequently it is not enough for man's happiness, that he become like God in power, unless he become like Him in goodness also.
Reply to Objection 2. Just as it is a very good thing for a man to make good use of power in ruling many, so is it a very bad thing if he makes a bad use of it. And so it is that power is towards good and evil.
Reply to Objection 3. Servitude is a hindrance to the good use of power: therefore is it that men naturally shun it; not because man's supreme good consists in power.
Source: Summa Theologica (New Advent)