Objection 1. It would seem that one man cannot be happier than another. For Happiness is "the reward of virtue," as the Philosopher says (Ethic. i, 9). But equal reward is given for all the works of virtue; because it is written (Matthew 20:10) that all who labor in the vineyard "received every man a penny"; for, as Gregory says (Hom. xix in Evang.), "each was equally rewarded with eternal life." Therefore one man cannot be happier than another.
Objection 2. Further, Happiness is the supreme good. But nothing can surpass the supreme. Therefore one man's Happiness cannot be surpassed by another's.
Objection 3. Further, since Happiness is "the perfect and sufficient good" (Ethic. i, 7) it brings rest to man's desire. But his desire is not at rest, if he yet lacks some good that can be got. And if he lack nothing that he can get, there can be no still greater good. Therefore either man is not happy; or, if he be happy, no other Happiness can be greater.
On the contrary, It is written (John 14:2): "In My Father's house there are many mansions"; which, according to Augustine (Tract. lxvii in Joan.) signify "the diverse dignities of merits in the one eternal life." But the dignity of eternal life which is given according to merit, is Happiness itself. Therefore there are diverse degrees of Happiness, and Happiness is not equally in all.
I answer that, As stated above (1, 8; 2, 7), Happiness implies two things, to wit, the last end itself, i.e. the Sovereign Good; and the attainment or enjoyment of that same Good. As to that Good itself, Which is the object and cause of Happiness, one Happiness cannot be greater than another, since there is but one Sovereign Good, namely, God, by enjoying Whom, men are made happy. But as to the attainment or enjoyment of this Good, one man can be happier than another; because the more a man enjoys this Good the happier he is. Now, that one man enjoys God more than another, happens through his being better disposed or ordered to the enjoyment of Him. And in this sense one man can be happier than another.
Reply to Objection 1. The one penny signifies that Happiness is one in its object. But the many mansions signify the manifold Happiness in the divers degrees of enjoyment.
Reply to Objection 2. Happiness is said to be the supreme good, inasmuch as it is the perfect possession or enjoyment of the Supreme Good.
Reply to Objection 3. None of the Blessed lacks any desirable good; since they have the Infinite Good Itself, Which is "the good of all good," as Augustine says (Enarr. in Ps. 134). But one is said to be happier than another, by reason of diverse participation of the same good. And the addition of other goods does not increase Happiness, since Augustine says (Confess. v, 4): "He who knows Thee, and others besides, is not the happier for knowing them, but is happy for knowing Thee alone."
Source: Summa Theologica (New Advent)