Summa contra Gentiles1.101 Chapter CI. That God is his own happiness

It follows from this that God is His own happiness.

For His happiness is His intellectual operation, as we have shown: and it was proved above that God's act of intelligence is His substance. Therefore He is His own happiness.

Again. Happiness, since it is the last end, is that which everyone wills principally, whether he has a natural inclination for it, or possesses it already. Now it has been proved that God principally wills His essence. Therefore His essence is His happiness.

Further. Whatever a person wills he directs to his happiness: for happiness is what is not desired on account of something else, and is the term of the movement of desire in one who desires one thing for the sake of another, else that movement will be indefinite. Since then God wills all other things for the sake of His goodness Which is His essence, it follows that He is His own happiness, even as He is His own essence and His own goodness.

Moreover. There cannot be two sovereign goods: for if either lacked what the other has, neither would be sovereign and perfect. Now it has been shown above that God is the sovereign good. And it will be proved that happiness is the supreme good since it is the last end. Therefore happiness and God are one and the same. Therefore God is His own happiness.