I’ve always taken studies about happiness with a grain of salt. As far as I know, what people call happiness is subjective.
Also, many times people jump to the conclusion that because happiness is in part genetically determined, or determined by what we eat, or by our environment, etc, there is little under our control that we can do to influence our happiness.
You can’t measure happiness directly. Studies about happiness measure a proxy instead. They define happiness, for purposes of the study, as a set of proxies. This “happiness”—quoted because it’s a particular understanding of happiness—is used to conduct studies and tests. This “happiness” may be the same happiness you understand when you say you are happy, but most probably it’s only a subset that may or may not overlap with your concept of happiness.
News about those studies about happiness, or the happiest countries to live in, etc., usually refer to happiness in general, forgetting to surround the word in quotes. When reading those studies—the actual studies, not the news about the papers—we should look for what the researchers were measuring, and how they defined “happiness” for the scope of the study.
The question remains, however: how do you define happiness? What are the proxies you are using for happiness?