“When it comes to life the critical thing is whether you take things for granted or take them with gratitude”, wrote G. K. Chesterton . Chesterton was one who understood both the gravity and the soaring joy of thankfulness. Gravity because giving thanks strikes at the very heart of what it means to be fully human; joy because giving thanks is necessary for being truly happy. In fact, giving thanks is perhaps one of the simplest and most certain ways to produce real happiness.
To see this twofold nature of understanding gratitude, consider these further Chesterton quotes. First, the seriousness of gratitude:
“The aim of life is appreciation; there is no sense in not appreciating things; and there is no sense in having more of them if you have less appreciation of them” .
The very aim of life is appreciation? If we understand appreciation as recognizing, enjoying, and properly responding to the good in something, then yes, absolutely appreciation is the aim of life. For the very purpose of mankind’s existence is to know God and to seek God as his ultimate end. And this includes love and obedience to God, which in so doing cultivates virtue within us, and from virtue flows true flourishing and happiness as human beings. Appreciation involves finding the good in things, in everything, and God just is the good of everything, since He is The Good Itself. God is man’s beatitude. And everything is good inasmuch as it flows from and is directed towards fulfillment in God. Recognizing, enjoying, and properly responding to this beauty and goodness in things, which reflects the Beauty and Goodness of their Source and Creator, is the very reason for which we exist.
So St. Thomas Aquinas says:
“All things desire God as their end, when they desire some good thing . . . because nothing is good and desirable except forasmuch as it participates in the likeness to God” 
From this follows Chesterton’s next quote:
“I would maintain that thanks are the highest form of thought, and that gratitude is happiness doubled by wonder” .
Thanks are the highest form of thought, because all thought is directed towards knowing, and the ultimate object of all knowing is again God Himself. To know the nature of something that exists is to know its Source and Creator, and the proper response to recognizing this is simply wonder and thankfulness at its very existence.
Again, gratitude is just the appreciation of things as they are, appreciation of the good in things. Unfulfilled desires are the cause of misery; so to be content in, and to rejoice in, things as they are, recognizing even the smallest flower petal as a gift full of more infinite goodness than we could possibly imagine, without heedlessly desiring more because we realize what we have been given is vastly more than we can even comprehend, is the surest road to happiness. To recognize that each blade of grass beneath our feet is the brushstroke of God as Creator and Being Himself is to open forth a gushing fountain of unending mirth.
In question 106 of the “Second of the Second Part” of his Summa Theologiae, Aquinas considers gratitude as a virtue under the cardinal virtue of justice. This particular question asks “Whether a man is bound to give thanks to every benefactor?” First Aquinas presents six objections to an affirmative answer. Then he responds with a single verse:
“On the contrary, It is written (1 Thess. 5:18): ‘In all things give thanks'” .
A larger portion of the same passage of scripture reads:
“See that no one repays anyone evil for evil, but always seek to do good to one another and to everyone. Rejoice always, pray without ceasing, give thanks in all circumstances; for this is the will of God in Christ Jesus for you” (1 Thessalonians 5:15-18, ESV).
It is no accident that giving thanks and rejoicing are placed together, because they are intrinsically interwoven actions. It is also worth pointing out that they are both commands.
Aquinas’s full account of the nature of gratitude is much longer, more complex, and more technical. But here we’ll give just a brief look:
“Every effect turns naturally to its cause; whereas Dionysius says (Div. Nom. i) that ‘God turns all things to Himself because He is the cause of all’: for the effect must needs always be directed to the end of the agent. Now it is evident that a benefactor, as such, is cause of the beneficiary. Hence the natural order requires that he who has received a favor should, by repaying the favor, turn to his benefactor according to the mode of each” .
In other words, we give thanks for gifts or favors received. And all gifts or favors are given by an agent, who is the cause of the recipient being a beneficiary of that favor/gift. And all effects are naturally turned/directed towards their causes, so every beneficiary ought to be turned towards his benefactor, the act of which is thankfulness. God is the First and Final Cause of all that exists, including man. Thus man’s “natural order” is to be turned to God in thankfulness. Writes St. Paul:
“For in him [Christ] all things were created: things in heaven and on earth, visible and invisible, whether thrones or powers or rulers or authorities; all things have been created through him and for him” (Colossians 1:16, NIV, emphasis mine).
God, in Christ, created all things for Himself, to be directed towards and to reach fulfillment in Himself. Elsewhere, Aquinas discusses how justice is the cardinal virtue of giving to things that which they are owed. And as a virtue “annexed” to justice, gratitude is lesser; because gratitude is all that we are able to give God in return for his gifts of creation and salvation, but the gratitude of man could never equal even a small, minuscule portion compared to the immense, immeasurable value and greatness of God’s gifts. And that is the meaning of grace. Existence and salvation are God’s gifts to man; but man could never hope to repay to God either of these things. Gratitude is all we have to give in turn. Giving thanks, in its fullest sense, is what man owes to God; but it is hardly a drop compared to the infinite, raging ocean depths of what we truly and totally owe Him. Everything is what we owe Him; our own finite thanksgiving is all we have to offer. And even that is already His.
So if you want happiness, if you want to be fully human, then “in all things give thanks”, for in doing so you grasp the very nature and proper order of existence.
. Summa Theologiae, I, Q. 44, Art. 4. Taken from online source: <http://www.newadvent.org/summa/1044.htm#article4>.
. See link for Chesterton quotes above.
. Aquinas, Thomas. Summa Theologica. 1265-1274. Trans. Fathers of the English Dominican Province. Coyote Canyon Press, 2010. ebook. II-II, Q. 106, Art. 3.