Here, Paul isn't just pleading us to be happy. He's not just saying, when things are going well, indulge! Look for the blessings! Be joyous when you can find something that makes you smile!
Instead, what he is commanding us to do is to CHOOSE to rejoice. In tiredness, in busyness, and even when life goes the complete opposite way that we would choose. Through rocky relationships, through changes we don't expect, in heartbreak.
Paul even says it twice. Rejoice.
Notice, however, that Paul doesn't say, "Work your best to be happy!" or, "If this doesn't come easily, you're not a Christian!"
Instead, he tells us to rejoice in the Lord, and always, at that. We are to make the choice to find our joy, our confidence, and our drive in our God. When all else seems to fail, we can rest in the arms of Jesus, and that is cause for rejoicing. When tragedy occurs, we know our God is infinitely good, and we can choose to find meaning in that. When everything is going well, our root of happiness should still be our God instead of our circumstance. Only then will we not be rocked when something changes or doesn't go our way.
Today, I'm choosing to rejoice. I'm not rejoicing in my cute house or the beautiful weather or even in my incredible blessing of a husband.
I'm rejoicing because my God is. I'm rejoicing because He exists and He is good. Today, and every day, that is enough for me.