The US "Declaration of Independence" document mostly authored by Thomas Jefferson in 1776 explicitly mentions it in the following famous phrase:
It makes pursuit of happiness a "Natural and Legal" right of every US citizen raising an interesting question "What is Happiness?" It is sort of assumed that we intuitively know what it is. Is it the hedonistic pursuit of pleasure? or "The calm contentedness of a sage?" or a "Hearty laugh at a joke?". It seems that the word "Happiness" may in fact have several meanings and it may refer to many mental states depending upon the context in which it is being used.
We hold these Truths to be self-evident, that all Men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness.
Harvard University has a long running study to learn the "Secrets of Good Life". The study is running for seventy years starting in 1937 and follows lives of its participants from the time they studied at Harvard till recent times. It is one of the longest running longitudinal studies.
The Harvard Psychiatrist George Vaillant is managing the work on this research for the last forty years. In the video from Atlantic magazine below he describes the findings of his work:
It is interesting to note from his comments that there is no single recipe that everybody can follow to attain happiness.
The complete article "What Makes Us Happy" from Atlantic magazine here, is worth reading. It is quite humbling in many ways describing how lives of people twist and turn as move through different stages of their lives.