I have the tendency to see the bias in such observations (which side is it that assumes from the get-go that their mood is immutable?). Nevertheless, I've decided that they probably have a point. Without some external stimulus to prod you out of your funk, can you really stir up some kind of joy just by thinking? We've all tried it and noted the artificial taste of manufactured emotion.
Hold on, though. I'm not turning in my Junior Ranger of Optimism badge just yet! I still believe you can choose happiness, just in a different way.
I say you can choose to be happy in the same way you can choose to be a doctor. At some point, you decide that's what you want to be. Then, comes the work. Becoming a doctor requires walking down a certain well-defined path, and it may be years before you get your diploma, but with dedicated effort your decision will result in a fully-realized dream.
So, there's a time-delay and an effort-contingency between the decision and the result. There's also the little matter of accreditation, by which I mean that there are literally millions of different voices in the world outlining a path to happiness. Most are well-meaning and effective to some degree, but some are blind alleys and dead ends like the 'doctor' who got his degree from CGNU online e-niversity. The path I use is the gospel path, which gives astonishingly robust results. Read more about it in Alma 32-33. Also, remember to exercise regularly.
Fortunately, rather than waiting over a span of decades for an MD, we can start feeling the fruits of our joy-hunting in a matter of weeks.
This all may sound obvious, but it's something we typically neglect. How often do we let our emotions govern our decisions rather than the other way around?
We may become truly free agents. Whatever path you choose, get moving on it because I guarantee that following no path at all inevitably leads to misery.
Let's get moving, then! Decide now that you want joy and don't let anything stop you!