Earlier quoted context omitted.
Sure, it's a choice in the same way that the outcome of an if-branch is a "choice". You have control over it in the same way a computer program has control over its own output. Which is to say: no actual control at all. Free will/choice isn't a coherent concept philosophically and I find it almost impossible to take anyone seriously when they propose "just change your mind" as a solution to anything. I am already str…
Not sure where you are going with the free will thing unless you are going for the genetics answer to happiness which I find lacking. Are some people happy? Yes. Are there people who were not happy who now are? Yes. How? Lots and lots of anecdotes are around changing how they approached life, not how they changed their genetics. Looking at your linked article, they are going off of a wellness questionnaire first deve…
The genetic heritability finding is not related to the free will argument, it is its own scientific finding. I don't know why the existence of those life-changing anecdotes matters at all, the research has never shown that happiness is 100% determined by genetics, only that the stable component of happiness has a high heritability. I also want to point out that "genetic heritability" doesn't simply mean "you don't got the happy gene, you're fucked", genetic inheritance is not mutually exclusive to environmental influences. Indeed, at least some gene-behavior studies will recommend "positive gene-environment matchmaking" [1] in its conclusions.
No happiness study is going to have a perfect definition of happiness, which scientifically is studied as "subjective well-being" because it's subjective. The study's definition of subjective well-being is reasonable and consistent. If you think that changing the questions will radically change results, I invite you to do or find your own scientific research. We don't exactly have a glut of happiness research, we have some modern meta-studies [1] that flesh out the relationship between genetics and behavior a little more but otherwise upholds existing heritability findings.
[1] https://link.springer.com/article/10.1007/s10902-016-9781-6