A survey showed (claimed) that Americans grow happier as they grow older. (I don't have the exact reference, or know the specifics, but the claim seems reasonable).
Line 6 (of 16) from Buddhism's Anapanamusmrti Sutra (discourse of the Buddha) is
"6. Breathing in, I experience happiness. Breathing out, I experience happiness."
I was struck, from both thoughts, that happiness may simply be a choice that people increasingly admit, or open to, even given all the inequities, discomfort, suffering and problems of our lives.
At some point we tend increasingly to chose happiness, perhaps simply by admitting that at some times we are happy, and by allowing that process of recognition and gratitude, e.g. for being alive, to blossom, even as we are aging or dying. Once happiness is admitted, we naturally tend our gardens, getting rid of the weeds resulting from our negative emotions, and nourishing our happiness to flourish.
Given Buddhism's teachings on Impermanence and No-Self, happiness may grow, hopefully in each generation as well as in individuals, but possibly not. It would take quite a study to measure whether humanity is getting happier, and what parts of it are happy, vs time.
Footnotes
The Anapanamusmrti Sutra includes lines suggesting ways to become more conscious of the breath, in the context of sitting practice for meditation.
Thich Nhat Hahn's book, "Breathe! You Are Alive", discusses this further, and gives its own exercises based on the Sutra. At one point TNH comments "Only when you feel happy can you have concentration. You cannot ... force yourself to concentrate." In more detail, he says that you do not want to repeat the words joy and happy without giving them a content, such as happy to be alive, and joy for the opportunity for meditation. He cites Buddha's examples, that joy is like, when lost in a desert, you see an oasis; whereas happiness is like, when you get to the oasis, you are drinking its waters. TNH's first five breathing exercises for following the breath, based on the Sutra, are to calm, stop, focus, concentrate, and nourish, saying further that without these elements we will tire of sitting (meditation practice).
We tend to scapegoat, blaming others or circumstances, and thereby maintaining unconscious "seeds" of anger, hatred, and jealousy. Even when we are happy we still retain our unconscious seeds, and if something or someone stimulates their recall we may become visibly and vocally angry, or hateful, completely dispelling our happiness. Most of this is unconscious stuff we carry with us.
Our balance of joy/happiness vs anger/hatred, may be our net influence on others, particularly those who are close to us, since we tend not to guard or temper our feelings when with them. In this way, if we are on balance unhappy, we probably spread the disease of our unhappiness to those we claim to love, those who are closest to us and whom we would most like to be happy.