Rejoice in the Lord always. I will say it again: Rejoice! Let your gentleness be evident to all. The Lord is near. Do not be anxious about anything, but in every situation, by prayer and petition, with thanksgiving, present your requests to God. And the peace of God, which transcends all understanding, will guard your hearts and your minds in Christ Jesus. Philippians 4: 4-7 (NIV)
What does rejoice mean? Merriam-Webster defines it as "to feel joy or great delight".
Let's plug that into Philippians 4:4; Paul tells us to "feel joy or great delight" in the Lord always. Always. Not just when things are good and He gives us the desires of our heart but also when we are in the midst of tests and trials. We can do this because we know that no matter what the day brings, good or bad, it’s not the end of the story. Our lives are so much bigger than our earthly desires. We were created for more than a good grade on a test or a job promotion. Drawing near to God and having a Kingdom vision allows us to rejoice in all things.
We can rejoice in the trials because they're shaping us to be used by God. We can rejoice in the blessings because we feel His favor in the time of rest. The biggest thing to remember when we feel tempted to not choose joy is that we are constantly in a battle. I don't know about you but I haven't seen any pictures of war where everyone was just chillin' on the battlefield having a picnic. Wars are intense times of struggle. Even during ceasefires there is a time of mental preparation. We often expect life to be easy and carefree but how can we have that expectation when we remember we are fighting, in a war, to make Christ known.
Don't forget that this is our temporary home. There is a battle for our joy here; a battle calling us away from God and towards sin. When we remember that we have not been forsaken and that we experience trials for His glory it becomes a little easier. He will give you strength when you have none left of your own and that alone is cause enough for joy.
Consider it pure joy, my brothers and sisters, whenever you face trials of many kinds, because you know that the testing of your faith produces perseverance. Let perseverance finish its work so that you may be mature and complete, not lacking anything. James 1: 2-4 (NIV)