I agree, it isn't a choice. Being happy or not happy is the effect of a cause. The method is what makes you happy or unhappy. You can chose to do actions that will make you feel happy or sad, but you cannot chose to be sad or happy.
For example, saying someone should just be happy when their Mom dies is extremely naive. Keeping yourself locked in your room won't help with you being happy overtime, but talking to family members and going out will good company will make you happier than sitting in your room.
That's one way to look at it. 🙃