To understand that happiness is really a choice, we could read the exodus where the jews enslaved for generations and Moses led them out of Egypt with gold, silver, and all sorts of valuables and when they reached the edge of Red Sea, what did they do? Were they happy?
They should be if they thank God for the freedom and all the treasure taken from Egypt!
They were not, because some of them had no faith in God and expected the worse to happen.
This choice, between being grateful for what we already have been blessed with versus having no faith or do not trust God and expect the worse to happen are the choice that will lead one to be happy and another be worried or grumpy even if they were in identical situations in life.
So, yes, it is a choice to be grateful or to complain or envy.