I agree in principle... in reality this isn't exactly true. While this doctrine (for this *is* indeed a doctrine) sounds very spiritual and empowering, it contains a logical fallacy.
Yes, ultimately we choose joy and peace.
There is also a very real and very powerful spiritual realm. We have an enemy who comes to kill, steal and destroy. He is given latitude to attack, isolate, break relationships... he will tear apart your friends, family, finances and faith. Ask Job.

We count it all joy when such trials present.
There is a difference between joy and happiness. God says: Be holy (set apart) as I am holy. He never said a word about happy.

In this world you will have many troubles, but take heart... He bestows His peace and He has overcome the world.