I love this. And need to remember this daily. I often (too often, actually) forget that I have so much to be thankful for and that I have absolutely no reason not to be anything but overflowing with joy. It obviously isn't always easy. But no one ever said that it was supposed to be.
The quote above says it so clearly--"choose". Sometimes our flesh doesn't easily allow us to naturally choose this disposition, but God has bigger and better plans for us. In choosing joy, we are strengthening our faith and hope in the Lord. This is a decision and not a mood--the state of our day, bank account, or clothing is irrelevant.
May the Joy of the Lord always be our strength, and may His Peace continually be our portion.