Objection 1. It would seem that not all desire Happiness. For no man can desire what he knows not; since the apprehended good is the object of the appetite (De Anima iii, 10). But many know not what Happiness is. This is evident from the fact that, as Augustine says (De Trin. xiii, 4), "some thought that Happiness consists in pleasures of the body; some, in a virtue of the soul; some in other things." Therefore not all desire Happiness.
Objection 2. Further, the essence of Happiness is the vision of the Divine Essence, as stated above (Question 3, Article 8). But some consider it impossible for man to see the Divine Essence; wherefore they desire it not. Therefore all men do not desire Happiness.
Objection 3. Further, Augustine says (De Trin. xiii, 5) that "happy is he who has all he desires, and desires nothing amiss." But all do not desire this; for some desire certain things amiss, and yet they wish to desire such things. Therefore all do not desire Happiness.
On the contrary, Augustine says (De Trin. xiii, 3): "If that actor had said: 'You all wish to be happy; you do not wish to be unhappy,' he would have said that which none would have failed to acknowledge in his will." Therefore everyone desires to be happy.
I answer that, Happiness can be considered in two ways. First according to the general notion of happiness: and thus, of necessity, every man desires happiness. For the general notion of happiness consists in the perfect good, as stated above (3,4). But since good is the object of the will, the perfect good of a man is that which entirely satisfies his will. Consequently to desire happiness is nothing else than to desire that one's will be satisfied. And this everyone desires. Secondly we may speak of Happiness according to its specific notion, as to that in which it consists. And thus all do not know Happiness; because they know not in what thing the general notion of happiness is found. And consequently, in this respect, not all desire it. Wherefore the reply to the first Objection is clear.
Reply to Objection 2. Since the will follows the apprehension of the intellect or reason; just as it happens that where there is no real distinction, there may be a distinction according to the consideration of reason; so does it happen that one and the same thing is desired in one way, and not desired in another. So that happiness may be considered as the final and perfect good, which is the general notion of happiness: and thus the will naturally and of necessity tends thereto, as stated above. Again it can be considered under other special aspects, either on the part of the operation itself, or on the part of the operating power, or on the part of the object; and thus the will does not tend thereto of necessity.
Reply to Objection 3. This definition of Happiness given by some--"Happy is the man that has all he desires," or, "whose every wish is fulfilled" is a good and adequate definition; but an inadequate definition if understood in another. For if we understand it simply of all that man desires by his natural appetite, thus it is true that he who has all that he desires, is happy: since nothing satisfies man's natural desire, except the perfect good which is Happiness. But if we understand it of those things that man desires according to the apprehension of the reason, thus it does not belong to Happiness, to have certain things that man desires; rather does it belong to unhappiness, in so far as the possession of such things hinders man from having all that he desires naturally; thus it is that reason sometimes accepts as true things that are a hindrance to the knowledge of truth. And it was through taking this into consideration that Augustine added so as to include perfect Happiness--that he "desires nothing amiss": although the first part suffices if rightly understood, to wit, that "happy is he who has all he desires."
Source: Summa Theologica (New Advent)