It's not a choice.
The path to become happy isn't instant though, it's much more complex than flipping a light switch, it's definitely harder for some than others (and possibly even impossible for some sadly), but for sure, you need to decide to become happy if you ever want to be.
Jordan Peterson, who for a long time I wrote off as a crackpot until I actually paid attention to some of his interviews and lectures, talks a lot about this. Happiness is a pretty crappy goal to have. It's a side effect of pursuing actual, worthwhile goals. People need to take some responsibility for their own state of mind and the things they are choosing to do that affect it.
It's a response. Internal responses, sure, but also - and more dramatically - to external ones.
> Jordan Peterson, who for a long time I wrote off as a crackpot
Trust your initial instincts ;)
It helps his cause that many criticisms of him are terrible and involve many fabrications.
This youtube channel contra points is a much better criticism then mine: https://youtu.be/4LqZdkkBDas
Fobbing that off as "well he's just being mean to us nice people", when they went after his career and means of supporting his family (at the time), doesn't cut it.
Also, worthy goals often bring pain. They often require you to forego own hapiness for something you consider more valuable. They often leave you burnout, feeling empty, unsatisfied and tired maybe even ressentfull. In particular, caregiving work is often like that. Soldiers also follow goals worthy to them (whether you personally agree or not with this or that goal) and are not happier in the result - it is called sacrifice for a reason. Those two are just obvious examples, not the only ones.
In addition to all that, your own happy personally worthy goal often brings unhappiness to somebody else, often to familly. It is extremely unfair to then blame that person for his or her own unhappiness since that has a lot to do with mine (or yours) decisions.