Last week, our Sunday School teacher had a couple of really interesting points while talking about Philippians 4:4. This is the one that reads something like this:
Rejoice in the Lord always; again I will say, rejoice.
Our teacher, who works at a HVAC company, was pointing out the difference between a thermometer and a thermostat.
A thermometer is something that measures temperature; a thermostat is something that controls temperature, assuming your AC is working properly, of course.
So how does that apply to Philippians 4:4? Well, the command ("rejoice!") is a thermostat-type command, not a measurement.
It means that we are not to find joy, passively, but to see joy in our being in God's will.
It means that our circumstances are able to control our happiness, but not our joy. Our happiness comes and goes; that's normal. I could have been happy if my team had won on Thursday; I can be unhappy that it lost. That said, whether I'm happy in my external circumstances or not does not affect my joy.