In honor of the Angelic Doctor, and in order to lead you to truly having a "happy" St. Thomas Aquinas day, I thought I would post some quotes of his on happiness. He begins by reminding us that we will not find true happiness in any created thing--no matter what next gadget or pleasure we attain, we will always be wanting more (emphasis mine):
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
St. Thomas Aquinas, Summa Theologica, I-II Q.2 A.8 body
As the good doctor stated, our happiness is found in God. Later he notes that happiness consists the contemplation of God:
Final and perfect happiness can consist in nothing else than the vision of the Divine Essence . . . For perfect happiness the intellect needs to reach the very Essence of the First Cause. And thus it will have its perfection through union with God as with that object, in which alone man's happiness consists.
Q.3 A.8 body
To reach the vision of God requires the rectitude of the will:
Rectitude of will is necessary for Happiness both antecedently and concomitantly. Antecedently, because rectitude of the will consists in being duly ordered to the last end . . . Concomitantly, because as stated above (Question 3, Article 8), final Happiness consists in the vision of the Divine Essence, Which is the very essence of goodness. So that the will of him who sees the Essence of God, of necessity, loves, whatever he loves, in subordination to God; just as the will of him who sees not God's Essence, of necessity, loves whatever he loves, under the common notion of good which he knows. And this is precisely what makes the will right. Wherefore it is evident that Happiness cannot be without a right will.
Q.4 A.4 body
We attain that happiness through living virtuously:
Rectitude of the will, as stated above (Question 4, Article 4), is necessary for Happiness; since it is nothing else than the right order of the will to the last end . . . Man obtains [happiness] by many movements of works which are called merits. Wherefore also according to [Aristotle] (Ethic. i, 9), happiness is the reward of works of virtue.
Q.5 A.7 body
In short: live virtuously (do good, avoid evil) in order to purify your will. (N.B.: Living virtuously requires us to ask for and receive God's help--we cannot keep up the life of virtue without grace.) The rectitude of your will will order you to the proper ultimate end (God), and you will love that proper ultimate end. Through your love for the proper end, you will move closer to it and eventually attain the end (union with God, vision of God as He is--eternal contemplation of the infinite Source of all goodness, truth, love, etc.). This is your ultimate happiness. This is what each of us is called to do--this is what we were made for. It is not easy; in fact, it is very challenging--it will take a lifetime of work, but that work will be worth it in the end. Doing that work will also have its own (smaller) rewards. Through living virtuously you will gain the satisfaction that you are doing what you were made to do--you will feel a real sense of fulfillment, not just a cheap high. You will be on your way to true happiness, not just fleeting emotional peaks.
Striving for virtue,