I don’t know that there is one universal answer. I can only speak for myself. And I seem unable, especially the older I get, to separate the concept of happiness from that of gratitude. And gratitude itself seems intimately tied to the notion of purpose. 

What Is Gratitude?
Gratitude, to me, in its simplest form, is the feeling that I am lucky more than I am unlucky in any meaningful way. More bad things can happen than good. This isn’t a simple comparison and contrast. I could, for example, get wrongfully arrested and spend the rest of my life in prison. That would lead to an immediate nosedive in my sense of overall happiness, of course. But, I suspect, because of the meaning I hold in my existence (more below), I would find a way to be happy even in such a circumstance. Gratitude, then, is about appreciating the good. It is easy for a rich man to be grateful, for example. That’s not what I’m talking about here. My gratitude derives not from material possessions or societal approval or even self actualization, I dare say. Outcomes cannot be the cause of happiness because outcomes cannot be controlled. Anyone who receives happiness from things he can’t control will find himself at the mercy of external factors. That is not happiness to me; it isn’t particularly useful either. Happiness derives from effort, from purpose. 

What Is Purpose?
Purpose comes from meaning. But is there meaning in the chaos of the Universe? I don’t know. Perhaps not. It’s a pretty big place and a lot happens, seemingly randomly. So, I am not talking about innate purpose, or meaning. There may not be such a thing. Saussure would approve. There might not be a God, or fate. It seems that way to me, most of the time. Therefore, I am talking about my own purpose here, derived from my own meaning. 

What Am I Grateful For? 
I am grateful for many things that happen to be in my favor; they don’t have to be so. In that way, I am lucky.

I am grateful to be alive; life is an opportunity to do things. These things have effects. Neither of these has to be so. What a wonderfully empowering thing! And, using reason, I may act in such a way that my effects are more good than bad, as I define them.

I am grateful that I have, and have shown, the ability to endure. I have suffered a lot of pain in my life; I was severely depressed for years for many reasons. But through considerable effort, I have developed into a tougher, stronger individual. It doesn’t always happen that way. 

I am grateful that I have in-built value to others, principally my parents. And that they tried more than they did not. 

I am grateful that my ancestors kept trying; that they survived weather extremes and famines and wars and pandemics and every thing the world threw at them; that I am a recipient of that effort. 

I am also grateful for the ability to learn from my mistakes, and, in that way, try to get closer to the bullseye. Ben Franklin said it well in his Autobiography, “. . . on the whole, tho' I never arrived at the perfection I had been so ambitious of obtaining, but fell far short of it, yet I was, by the endeavor, a better and a happier man than I otherwise should have been if I had not attempted it . . .” 

I am grateful that there is beauty when there need not be, that I can appreciate it. When I look at nature, I see myself, a product of natural laws working relentlessly for eons to make slightly better adapted things. 

But, truly, my gratitude is not about looking at the cup as more full than empty; it could even be more empty, for that matter. My gratitude stems from my purpose, regardless of whether or not there is a God, or fate, regardless of whether or not I have succeeded. 

What is My Purpose? 
My purpose is to help others live lives with less necessary suffering, or, in short, to paraphrase Benjamin Franklin, to make the world a better place for my having been in it. Maybe it sounds cliché. But there’s little more that makes absolute sense to me. Think about it. If the Universe has no meaning, which seems a strong possibility, this dictum would still carry weight. Let’s explore. Suffering is, by definition, not good. I do not mean meaningful pain. That can be sacrifice or growth. I mean pain that causes a lack of meaning, such as someone who is a devout Muslim being force-fed pork. The subsequent suffering here is a result of, I argue, an inability to live meaningfully for this person. Of course, suffering depends on a person’s understanding. He might reason that his suffering brings him closer to Allah. Fine. But the suffering most Muslims would feel here is due to a breakdown of the values - the meaning - they hold dear. Let’s go further now. Suffering is part of life. It’s not going anywhere any time soon. And my existence, like all those who came before me, is finite. And I am but one among a seemingly infinite number of human beings. Therefore, if I can do something, even if it requires my own suffering, to make it more probable that the world in which my fellow humans live in is such that, as a result of my existence, is less likely for more people to have more suffering, then that is good - absolutely. And I have the ability, and desire, to live my life trying to do that.  

What Is Happiness to Me? 
Happiness is this feeling: that I, who need not exist, however imperfect, am, on net, a positive force acting on the world, which itself need not exist; that good exists at all and that I am alive to witness its magic. Any sufficiently good act is indistinguishable from magic.